ive had a male betta in my planted 20G high for a few months, and havent had any problems yet.
he's in with:
4 red rasboras
3 black khuli loaches
1 black red-tailed shark
1 common plec
1 ghost shrimp
the plants also probably help, as they give a bit of hiding areas, occasionally the betta will chase the rasboras or the shark will chase the betta, but they havent actually done anything to each other.