Hello! In answer to your questions:
You can use anything that you're legally allowed to use. It's up to you to check whether or not the creator of those assets has given permission. So whether or not you're allowed to use a tileset you found on an RPG Maker forum comes down to whether the creator of that tileset allows people to use it in their projects.
No, covers of songs are not allowed. The only exception to this would be if you actually obtained a license to use the cover song in a game.