Okok I'm 99% sure I get this and you guys aren't.
Pokemon you can use in RBYFRLG: None until you beat the game or trade/hack someone in
Pokemon you can use in GSCHGSS: Any of the original 151 Pokemon that appear in your game. So, Pidgey, Rattata, Geodude, Jigglypuff, Bellsprout, etc. Or trade/hack someone in.
Pokemon you can use in RSE: Any of the original 251 Pokemon that appear in your game. So Tentacool, Geodude, Zubat, Slugma, etc.
Like that. Honestly, I'd say if you are playing in Red/Blue, you can only use Pokemon that get new evolutions in later games. So like Poliwhirl (but not Wrath), Mr. Mime, Scyther, etc.