When it comes to Bettas in a community tank, you always need a contingency plan just in case things don't go as planned. (i.e. spare tank) Sometimes they are fine other times they are a terror. There is also the possibility that the fish you chose could injure or harass your Betta as well.
You don't want any fin nippy fish, fish with flashy or flowy fins as Fluff mentioned or other anabantoids like lmw80 had said.
Otos are usually a good start, some apple snails though they may mistake their antennae for worms in which case they couldn't be housed together, sometimes ghost shrimp, neon tetras, cories are usually ok. I have one with ADFs (African Dwarf Frogs) but keeping the frogs well fed has been a challenge with a Betta in the tank. I tried a Betta with Danios once and things were fine for several months, then the danios began nipping my Betta's fins and he had to get his own tank alone.
good luck.