If your going to add anything, make sure you have these:
a hide for the betta ( or a place it could hide), for when It wants to get away
proper heater and filter that work with the betta and other fish
I also suggest no fish that are really bright in colours ( because the betta my mistake it for another, and try to kill it) and fin nippers, like barbs.
Fish that would work good, that I have paired with then are 5 zebra dainos, you dont want less then or more then 5, because of the active level, And kuhli louchs, about 5-7 they burrow, and are nocturnal.