I think we'll allow 3 characters a player, for those who want to do as much, but otherwise I agree; the intelligence of the animals would likely make them see each other as somewhat bretherin, and having them work together would be interesting. For example, whilst a big gorilla might be able to tear a zombie limb from limb, he aint gonna be able to go through that air vent and open the locked door from the other side. Enter bionic bunny, with its eye-mounted laser cannon and the ability to hop fifty feet into the air. 

Righty'o, I think I'll dial it down a bit, keep it from getting too complicated and convoluted. The beta will pretty much be the extent of it, I just need to make a mention of the third threat: human survivors, who will never pass up a chance to shoot, kill and cook something.