I have a situation planned where the players will playing as monsters, using the stats in the monster manual (entirely in lieu of their own). Are there any guidelines anywhere about how to balance encounters between monster groups in this manner?
Note 1: This is not the same question as level/CR transparency; during their time as monsters they will all be monsters, and encounter no 'heroes' with character levels, so I don't care how PC monsters balance against other PC's, only how I can balance them against other monsters.
As a rough rubric, I assume that working out the CR of the PC-monsters, then preparing an encounter with that CR would be roughly "balanced", as in either side is roughly equally likely to win... but that's not how the game is played, the players usually win encounters (with some degree of risk and attrition), and have an advantage in the forms of abilities and options available to them, I'm just not sure how to adjust for the lack of those...
Note 2: I'm hoping to allow players to pick their monster forms freely (or at least restricted by a maximum CR), and have them face a variety of challengers - I'm aware this will certainly result in situations requiring my adjudication regardless of balancing rules used (abilities like a lycanthropes resistance to normal weapons for example will be very strong vs many monsters), but I'm hoping to minimize this as much as possible going in; I am ok with heavy-handing things where required, but would prefer to keep that to edge cases rather than constant moderation...
I'm open to official rulings I may have missed (I'm sure there are rules I've read but not absorbed in the core rule books, and I don't keep up with the living errata on twitter), or any third party/homebrew suggestions on how to balance this.