Okay, first of all, I'd like to share the benefit of my experience trying to organise an exotic type game.
Not wanting to put you off, because I am definitely interested, subject to what actual creatures are decided...
I think that TheHangedMan's idea of having divisions within the two tribes is unworkable in a practical sense. To make it work you would need at least eight players, four on each side, two each of a species... That is the minimum.
You would be far better to either specify two species, or simply allow people to pick whatever species they liked and then divide them up into tribes based upon some GM's arbitrary decision. If two players happened to pick the same species that would be pure coincidence, and they need not necessarily be on the same tribe!
In fact, you could do forest dwellers and field dwellers as the two tribes, or something like that.
If you get enough players, you could go with more than two tribes, but my advice would be to Keep It Simple! The more complex you make the plot, the harder it is to keep people interested, and the harder it is to keep it going. Plus you make a rod for your own back!
Anyway, that's my advice, for what little it's worth, speaking as someone who has watched all but one of her own group RPs fail (not including the one I restarted in the ones that didn't fail). What you do with it, is your choice.
I will still be interested if my advice is not followed, just so you know! *grin*