I don't really like your idea. Perhaps for gym battles/ "official battles only? If you think about it, pokemon is about having the team you want, and cutting in on your ability to have a team you want is wrong, especially if you want trainers to not fight you if you only have one pokemon and they have more. I'm assuming you meant they will only use as many pokemon as you, but that still is ridiculous. Some later teams literally rely on having more than one pokemon to carry out a strategy. It would really restrict most people, and if anything, it should be an option, but not tacked on to a hard mode. In my opinion, the worst kinds of games tack a gimmick to the hardest mode, forcing gamers who enjoy hard mode to go through something they rather wouldn't.