I have a clip from the Nintendo Switch, and I posted it on twitter. Should I download the vid and put it on YouTube or is twitter fine?

You need to ask the mods of the game to see if they have any rules regarding where video proof is hosted (or you can check the rules of the game too).

Would strongly recommend using YouTube instead of Twitter, but for most games Twitter is probably fine. As always and as @Timmiluvs said, check the rules for whatever game(s) you plan on running.