I really feel this point needs to be made, if people disagree I will take the down-votes. I feel is answer is necessary because many future readers might get the (incorrect) impression that mixing templates in WoD is a great idea. It isn't. It is cool in other systems (DFRPG, for example) but it isn't here, and this is why and here is my recommendations. I am focussing more on mixing supernatural templates, which to be fair is just a symptom of the un-unified party concept.
In short, don't. I know that isn't what you want to hear. But let me have my reasoning.
You say "Implicitly I think the mix doesn't /have/ to negatively impact things, just means a different type of game."
I disagree, without very experienced GM and players, it will negatively impact the game.
Even with them it will be as you say a "different type of game", it will not be a the kind of experience one expects from the world of darkness.
The fact that you are having an issue with keeping the characters together, indicates you/your players don't have that experience. That is Ok, there is nothing wrong with that.
This cross over that is not designed for, is going to significantly aggravate the issue of having a party that doesn't have a strong reason to be together.
I am playing in such a game. I would not chose to every GM such a game.
This game has taken extreme organisation. It had pilot games run before hand to test the waters to see how things would go.
The GM and all of the players each had over 3 years of experience with nWoD. All the players bar 1 was an experienced GM, who had GMed several nWoD campaigns.
Vast amount of story prep went in before things started, weeks I think.
And even now after over 18 months of weekly games, hours are spent some weeks cross referencing rules for their interactions.
The GM needs to know the rules and fluff for all the templates, at once.
That is keeping a lot of information in short term memory.
For reference, how we ended up working together was we were all metaphorically sold to an archmage. Someone us mistakenly sold ourselves by entering his employment, others were sold out by our own supernatural organisations. He used imperial magic to bind us to a Gies forcing us to work together to solve his problems. By the time we were individually strong enough to break the gies, we had now long relationships with each other.
I have also run a game entirely around the premise that once player was a vampire with abilities to imitate a werewolf, and the rest of the party was werewolves. This worked ok, because it was clear we were playing a game of werewolf. No one except me and the vampire player knew he was a vampire. The campaign ended when he admitted to the party (In Character and Out) that he was a vampire -- after getting a oath from them that they would all help deal with each other's problems. This was what the story was about; even though no-one knew what was going on til the end, once the realised this it all lined up. In this game it would not have been fun to continue for more sessions after that.
Being able to mix PC supernaturals was not a design goal of nWoD. The fact that is it possible is a side effect of of the design goal of allowing NPCs from different templates to show up.
In oWoD this was not technically possible, but could be done with some minor house-ruling.
Each supernatural book was stand alone, and was a slightly different system for each with subtle differences, from what I have heard.
(I should actually ask that as a question, I am just going on second or third hand information with anything to do with oWoD)
The interactions have some issues, both with fluff and from a balance point of view.
They also all bring in there own dozens or even hundreds of NPC strong organisations that for some are a major point of playing them. In particular Vampire is all about this, it is game of politics. Changeling can be very similar. Hunter to a lesser extent within a conspiracy/compact they are fairly united, but again it can be a thing in hunter, either internally with factions or externally.
Managing a bunch of these factions is not going to be great as a GM.
It will be a lot of work
As a Compact Hunter the Null Mysterium will have less NPC faction baggage than a hunter with a conspiracy. However he will have no mystical ability, no "Magic" endowments. So while the vampire is dominating peoples minds, and the changeling and werewolf are off in there own private dimensions, he is going to be taking notes. This isn't super fun for a player. Without a cell of hunters he can't use any cool hunter tactics either.
Hunter is also meant to get practical XP. Assessing practical XP needs to be done every single scene that contains a supernatural. That is going to be all of them.
The hunter should be getting significantly more XP than the other characters (maybe 1.5x as much).
This will help deal with his low power of not having sweet abilities, though won't really help him have more variety.
It may also make the other players (out of character) uncomfortable, and feeling like they are missing out.
The Hunter lacks a Power stat (this is why hunter is a Minor template), he is human.
As such he does not get to add power stat to resist super-natural effects.
This is not too noticeable when the rest of the party is at power stat 1.
But when they are at 3, then he is having to spend a willpower point to do what they can just do. In a normal Hunter Game, the hunter could use the Moral Support Tactic (Page 226), to boost their effective resistance, but that only works if you have a party of Hunters (as I have been saying, hunters are not meant to work alone).
Further, he is human for purposes of effects that affect humans.
In particular lunacy: page 176 of Werewolf, and page 319 of Hunter.
In any scene that the werewolf transforms the Hunter is going to suffer debilitating penalties, may be unable to remember the scene, and may even be forced to flee. (Depending on Willpower (ie Resolve+Composure total). The Hunter does not even get a Roll to save against this.
Normally this would be an aspect of horror in the occasional werewolf scene in hunter. but with a werewolf in the party, this is going to be a very large portion of the scenes.
Secondly from the hunter being Human, he counts as a sleeper for purposes of the mage dealing with paradox, and disbelief. This will to an extent nerf the mage, but will mostly just make things more chaotic, with the mages spells misfiring (Havok), and random other paradox effects.
I recommend house ruling this out of the game. The fact that you have to, indicates the designers never intended these two characters to be part of a team.
It is a Harmony 6 Sin to not be around other werewolves (Page 183).
Though they can survive some weeks, depending on there Primal Urge.
It is a Harmony 4 Sin to reveal the existence of werewolves to a human (Page 183).
It is debatable as to if a changeling counts as a ordinary human.
But a Hunter definitely does.
Vampires can't act during the day.
This is a big issue.
Even leaving aside
Depending on the time of the year and location, a Vampires can only act for ~8-16 hours,
while the sun is down. This is a average of 12 hours.
vs all other characters who can act for 16 hours per day, and if they want can go without sleep for 48 hours. Vampires just can't.
I worked around this in my vampire hiding amongst werewolves game by having the pack's Totem's ban be going outside during the day. The totem spirit was aligned distantly, to a spirit involved in the creation of vampires, and was in on it (there was alot more fluff that had the players accepting it without blinking).
Mage does not have a power curve similar to the others.
When a mage increases his Arcana, he doesn't get one more spell -- like the changeling does when he ups a contract,
he gains a theoretically infinite number of spells. Uping an Arcan
Mage allows improviseings new spells -- including even if he is not creative,
Indeed almost all spells in mage will be improvised, since learning rotes is really hard without other mage NPCs, and also is broadly speaking not worth it.
The mage will not have the most powerful abilities -- generally mage abilities arrive at one dot higher than the equivalent ability in any other supernatural.
Though there are exceptions, for example 1 dot of the Spirit Arcana gives almost every ability in the Promethean Spiritus set of Transmutations.
However he will be the most versatile. Especially as he levels up.
With 1 dot in a arcana, that give him sight abilities, he will be able to work out what is going on better.
With 2 dots in a arcana he has some control abilities that will let him solve problems -- 2 dots in any randomly chosen 4 arcana can solve most problems.
When he hits 4 dots in a arcana, depending on the arcana he will be able to solve problems in special ways.
Such are turning all the air in a room in to chlorine to nerve gas everyone,
or using Prime to create any simple object.
His versatility will really start to show up the hunter's lack,
especially as the hunter isn't getting any special powers being that he is tier 2.
Mage is also the most complicated system.
It is good that it is explicit about so many things, but it does mean that you will be spending alot more time checking up on rules for the mage player than for anyone else.
Evidence that Mixed PCs was not a design Goal.
As I said above, I believe nWoD was designed to allow NPCs to cross over between.
PCs can, merely as a side effect.
I am yet to find word of god, saying one way or the other,
so this evidence is circumstantial.
Crossover abilities are not found in core books
A few rules do exist for facilitating cross over, but not many. Most of those I would argue are much more suited to either single character insertion (Eg one vampire hiding in a pack of werewolves), or for NPC stalking (Party infiltrating a faction of another supernatural).
All these special abilities that facilitate cross-over, are in Splat books for the templates, such as:
- Lord Sage of Unknown Reaches for changeling (Lords of Summer).
- Refinement of Silver for Promethean (Magnum Opus).
- Dead Wolves Bloodline for Vampire(Shadows of Mexico)
I am sure Hunter has rules for "working with the enemy" but I don't know where.
Point is none of this stuff is in core for any line.
If it was a design goal of the game for cross over to be a thing they would be in core books.
Different XP for established characters
- Rank neonates 0 experience points
- Up-and-comers 35 experience points
- Established Kindred 75 experience points
- Movers and shakers 120 experience points
- Elders and other linchpins of undead society 180-300+ experience points
Changeling, Werewolf and Mage have:
- Fresh 0 experience points
- Established 35 experience points
- Veteran 75 experience points
- Lords 120+ experience points
Roughly one step earlier than vamp. Meaning that vampires continue to develop for longer, and take more XP to become established. Still not a huge thing, mostly says that vampires of significantly higher XP than other high XP supernatural are more common (a shiften distribution).
Hunters on the other hand are completely different:
- New Recruits 0 experience points & 0 Practical Experience
- Survived First Contact 25 experience points & 12 Practical Experience
- Seasoned Soldiers 60 experience points & 30 Practical Experience
- Grizzled Veterans 100+ experience points & 50 Practical Experience
A reading of that indicates that Hunters have a very different development curve to the others; and that this is by design.
Thus the designers did not intent to mix hunter with the others as they level at different rates.
While it is unquestionably possible to mix and match templates, it was never the intent.
My suggestion is:
Run with your story, with these characters. It sounds like a fun enough seed.
If you don't have much more the other answers provide good suggestions.
Get one session out of it, or even 3 if things are still going.
It won't go well forever.
As you correctly say these characters have no reason to be together.
Once they all split up, end this Story.
Now you have some more experience with the various different supernaturals,
and can as a group decide which supernatural you want to play.
And while you are creating new characters of that type,
you can ensure all the party members have a reason to be together as a group.