I don't think we should even take Arceus into account as what makes it change its sprite+typing is the item it's holding. It seems to me it would be the same as treating Specsmence and CBMence as different pokémon. Same thing with Unown, as its only difference between formes besides the PID thing (lol seriously) is the sprite, and no way in hell we're going to let people use blue and green Salamence at the same time.
I agree that our Species Clause should allow alt. formes to be used on the same team, as they're pretty different on their own. None of them share the same base stats except for the five Rotom-App, and even then each of them have different typings in B/W.
Originally Posted by Fat R_N
"It's a mess is what I'm saying" - Slogan of GameFreak