i like this idea, i really do. in a perfect world we'd be able to do this. it can theoretically open up some new strategies—one might want more than one form of, say, ogerpon or slowking or even deoxys on their team—while seemingly not inviting anything actually broken because you're usually either weakness-stacking on some level or just running something that isn't that good.
however.
this is in fact not a perfect world, as evidenced by the fact that i don't own a submarine, and so there's a very large and obvious problem that we will inevitably run into: not every distinct form is mechanically different. for example, you'd be able to use zarude and zarude-dada on the same team because they're legally two different forms, even though they're exactly the same mon in every respect aside from one wearing a scarf. this might not seem a problem to some, but it's becoming increasingly common for game freak to give these alternate forms to mons. this gen alone, there are 2 dudunsparce forms, 2 maushold forms, and 4 squawkabilly forms, all of which are almost identical (the squawkabillies get different abilities but they're still close enough to mention), plus some other things i'm sure i forgot to mention. we'd have to open up a discussion on what forms of alcremie even count as "forms" and even if we chose the bare minimum you'd still be able to run a full team of alcremies. even if we decide to not do anything about alcremie, you can run a 6-pikachu team in whatever tier pikachu's in because they keep giving the rat bastard a billion new hats every gen. and these are just the problems we have right now. what happens if next gen comes around and they give identical forms to a good mon, or worse, a great mon? say something that's, like, iron-valiant-tier good drops next gen and there's 4 mechanically identical but cosmetically different forms of it. what do we do then? we can't go picking and choosing what forms "count" under species clause, it's all or nothing. it's better to just keep species clause the way it is; the potential dangers outweigh the potential benefits of reworking it
also the fact that it's an actual in-game rule you can implement