As for Ork clans, we're not at all sure as to which one we want to include. Since the mod is taking place in the Armageddon campaign, the same clan that Ghazkhull led is a must-include. However, I think I heard that there were different clans involved, but that's something that needs looking into.
Clans are really ethnicities rather than social groups - any given Ork tribe is composed of multiple clans, so you can pretty much just go with whatever your artists like (or pick a different clan per class as appropriate). I have to ask though, why are you going with Orks as antagonists rather something more similar physically? Renegade Guard, Eldar, Tau, etc. You'd be able to share a lot of animation assets between the sides (which you're going to struggle with anyway - all mods seem to - since good animators with spare time on their hands are like rocking horse ****