Different version wise I was more thinking the later versions of the base pokemon game
Competitive Pokemon is divided up into different metagames depending on what generation you are playing and what Pokemon are allowed. When you play in an older gen, you play with all of that gen's idiosyncrasies, and they sometimes become a big part of the strategy. For instance, in gen 1, Hyper Beam is exceptionally powerful because it doesn't require a recharge if it knocks out the opposing Pokemon, so using Hyper Beam as a finishing move is a big part of gen 1 strategy even though it got fixed in all the later games.
Stadium is kind of gen 1.5; it's gen 1, but it has some fixes, including some significant ones (like fixing the aforementioned Hyper Beam quirk), so Stadium is really its own metagame. However, it's not represented on Smogon's Pokemon Showdown battle simulator. That's why I'm making the hack: so people can play the Stadium metagame. So changing the rules to reflect stuff from future gens would kind of defeat the point.
At some point in the future, it would be interesting to port some of the other changes from future games (like updated move lists, the physical-special split, and extra types) to see how it impacts the metagame, but it's not something I'm planning at the moment.
I also recall many years ago the random move... move I think it was mewtwo had also included waterfall or something in the mix which did not appear in the main list of random moves in the main game, or if it did it was so rare compared to every few moves we saw on stadium. Don't know if it was just something like gambler's memory but I do wish to note it.
I'm not sure what you're referring to. Mewtwo doesn't learn Waterfall in any of the main games, and neither of the CPU Mewtwos in Stadium
use it, either. Plus Mewtwo isn't a rental Pokemon in my hack and Waterfall isn't a particularly useful move, anyway. In gen 1, it's just a less powerful version of Surf, and it can only be used by Goldeen and Seaking.