If I were to give the store some PS2 and Wii games, would I be able to use game credit or trade those games for 3Ds games, or does it have to be from the same console?

Yes! Store credit works for anything, no matter what you traded it in for. For instance, you can trade in a ton of games from the Wii and put it toward new Switch games, Sega Genesis... Anything.

No it doesn't have to be for the same console. Level 7 gives you the choice. You can receive a fair cash amount or a bit more in store credit. You can trade in PS5 games for original Nintendo games if you want.