I'll be honest, I have no idea how I stumbled upon this post from a month and a half ago, but since I've just so happened to wonder about this myself I wanted to toss in my input that no one asked for. All jokes aside, I don't know much about the technical side of battle simulators nor do I know much about these commands, but what I can say is that Custom Games should be able to run newer Pokémon in older generations with not very many issues, at least on Pokémon Showdown. The idea that certain Pokémon and strategies could be absolutely busted if they were introduced in a different, earlier generation (for example, Slaking and Regigigas being good in Gen 1 because of Normal typing and no Abilities) has always fascinated me, and while I would imagine such metagames wouldn't see a whole lot of action, I can definitely get behind the appeal of players wanting to experiment with cross-generational mechanics and combinations of mechanics. Unfortunately, for the sake of RBY Custom Games, I don't know of any way to set which Special stat (Sp. Atk or Sp. Def) would be the properly chosen Special stat for the Pokémon's hypothetical RBY counterpart. To my knowledge, the current system with no commands used defaults to having the newer Pokémon's Special Attack as their RBY Special Stat, which is good for Pokémon like Espeon my beloved but bad for a Pokémon like the 100 Sp. Atk but 200 Sp. Def Regice. I'm sorry I can't really help much here, but I do think you and your friend group could play some RBY Custom Games while you look for a more permanent answer to your questions. :)