I once had a bad experience with bettas and white clouds. Guess who won?
However, I can tell you *IME* that platies, neon tetras. swordtails, african dwarf frogs, apple snails, and corydoras are wonderful tank mates for bettas. I think plecos and loaches would also work, but I have no experience with them.
One popular fish that hasn't been mentioned yet is the danio-- I would avoid mixing them with bettas. They are too fast and nippy. Also avoid any other type of labyrinth fish-- including gouramis, other bettas of either sex, and paradise fish.