Enforcing our version of species clause would not be changing in game mechanics, so really, the results of that thread and this thread are not correlated.
That being said, although I think it would be nice to allow multiple formes, Eric is right in his listing of some pokemon that arguably would have to be allowed to be used if we decided to do this. I was trying to come up with a rule that would allow multiple formes of the same pokemon, to be allowed in battle, but was unable to, due to my (perhaps arbitrary) decision to exclude Arceus from this list, since the two guidelines I thought about including involved changing Type or Stats, but Arceus messed with this, as well as the Shaymin-s changing back to land forme when frozen (which is lame, but whatever).
It probably would be easier to just continue using Nintendo's definition of species clause, while tiering the formes of Shaymin, Rotom, Wormadam, Dexoys, and Giratina separately. I would still like to remove that, but I can't think of an easy way to.