In a weird way, the problem is actually easier in this case. To begin with, we can examine: what makes wizards a balancing problem when the party does include wizards? How do Wizard PCs balance against non-wizard PCs?
It's inherent to the system that wizards are not just powerful but flexible. Wizards can use their power for a wide range of different situations, whereas most other supernatural powers are more situational. This is important in FATE, where flexibility is a huge advantage.
The balance for this lies in part in the fate point economy. Mortal characters have a much higher effective refresh, and correspondingly greater power to change their fate.
Non-wizard supernaturals, on the other hand, are rarely capable of a comparable range of effects, but are usually more powerful and much more reliable at doing the things they can do.
So: There are balance differences with a non-wizard party, but a lot of compensation is already built into the system. The total refresh of a party matters a lot more to their power level than exactly how that refresh is spent.
With that comparison in mind, we'll look at some specific strengths and weaknesses of wizards:
- Weakness: Refresh. Wizard PCs have few fate points available; NPCs should show a lack of control reflecting that. Don't give them a lot of fate in hand. Make sure they have exploitable aspects. (This is still true if there are wizards in the party, of course.)
Strength: Flexibility. A wizard's big advantage. Evocation lets you cover pretty much any combat action.
The thing is, barring a large skill advantage it isn't as good as it seems. Without taking a lot of stress, it's hard to get an unbeatable amount of power out of evocation. A wizard can be treated like anyone else holding a big dangerous gun: take cover, but don't panic.
Weakness: Endurance. A wizard's combat power is limited by their ability to evoke; when they run out of spare mental stress they get less dangerous. Let the party make plans to drag the fight out (or catch the wizard when tired from dealing with expendable cannon fodder...)
A smart party with mental attacks can exploit this, of course.
Weakness: Lack of resilience. Wizards simply aren't as physically tough as other supernaturals, and don't generally have skills and stunts to spare for many combat skills.
Of course, if the wizard is another type of supernatural, that's a whole other level of nasty. (Mavra is dangerous!) Don't go there until you've got the hang of the balance with a 'standard' wizard first.
Weakness: Numbers. Wizards don't generally come in large groups. Unless you have a very large advantage in skill level, it's very hard to raise a defence that will block strong attacks from several people, and still have actions and evocations to counterattack with. So dealing with a wizard in a simple fight isn't actually particularly difficult - just cooperate to get attacks through from a number of different directions.
Strength: Preparation. If the wizard is expecting a fight, none of the above weaknesses apply. A prepared wizard can deal with them. I'll discuss this in more detail under 'thaumaturgy' below.
Strength: Contacts. A full White Council wizard (or That Other Group, he said in a vague and hopefully spoiler-free way) has a lot of contacts and friends in the magical world.
However, this isn't particularly big problem for balance; it can be handled the same way as any other well-connected opponent. It may also represent an avenue of attack, of course.
Strength: Magic items. A wizard can easily craft focuses that shore up weaknesses or enhance strengths.
This isn't a problem, as it actually balances out fairly well against stunts / skills / spare refresh. The fate point economy already takes this one into account.
Strength: Planning. A wizard can almost always achieve his goal, given time.
This is the big plot problem to be aware of. Avoid plotlines in which an NPC wizard is planning to kill a party member directly, or other such goal. Instead, allow the party to interact with the wizard in such a way that the wizard is the focus of their attention - but not necessarily the other way around. Don't make them deal with a fully prepared wizard unless they've had a chance to find a weakness to exploit or a 'busy time' to attack. (Note that even Dresden usually wins against wizards when they're busy with some other thing and can't interrupt.)
Strength: Thaumaturgy. This is the number one issue that you must address. In Dresden Files a prepared wizard can build up a truly ridiculous action bonus to produce a spell that simply can't be countered effectively. (The example in the book covers how to produce the 'instant death, no survival' spell from Storm Front, after all.)
The thing is, that's just as true if the party does have a wizard - a major prepared working will still blow through any defence the PC wizards can throw up with a hasty evocation. The standard advice on how to handle this still applies:
Remember that thaumaturgy needs preparation. Give the players chances to find out what's coming, to interrupt the gathering of ingredients, or to simply attack before the ritual is complete.
A wizard can't prepare for an attack he's not expecting. Let the players naturally happen onto, and interfere with, the wizard's plans, instead of being a threat he's aware of in advance.
Strength: Thaumaturgy. Yes, I already said that. But it goes both ways. The biggest weakness of a non-wizard party compared to one with a wizard is the lack of ritual power. Without that option, the party can't build a single killer trick-up-their-sleeve in the same way.
Compensate for this by making sure they don't need to. Don't protect the vampire's lair with a single massive barrier to overcome (unless you've also provided some C4); substitute a number of weaker walls. Don't use single very-tough creatures without weak spots, because the party has less ability to research a 'killer punch' - again, use more weaker creatures, or an opponent who can be beaten without requiring a single massive skill total.
Addressing your specific party: It sounds like your party still have a flexible range of magical punch even without the actual wizards. (Psychic grimoire? Awesome!) If you find you need to give them a little extra magical cover because they're losing horribly, the Emissary should provide a convenient plot hook for emergency help from something bigger... and an appropriate price tag.