Tank mates completely depend on your betta's personality. I have three bettas in larger tanks at the moment, so I'll discuss them to give examples.
I have Fugu, my dumbo halfmoon in a 27G cube with threadfin rainbows, a mild-mannered German Blue Ram, 6 otocinclus cats, and about 30 red cherry shrimp. He hunts the shrimp a LOT. In fact, during much of the time he spends at the front of the tank, I only see his bottom half. He dives head-first into the plants to peck at or hunt for the poor shrimps :P Once I'm out of cherries, I don't think I'll buy anymore.
I have my mustard gas in a 15g with furcata rainbows and otocinclus, and he does rather well. He's probably the most mild-mannered of all my bettas, as he even lets the rainbows get food before himself. I usually have to feed him specially out of my hand or tongs to made sure he gets enough food. :P
Then I have Sashimi in an 8g cube. I thought that surely he'd tolerate otocinclus, since they're algae-eating cats that suck on plants and the sides of the tank (not invading his swim space), but that was unsuccessful. I tried giving him some otos, but I found that he mauled one, so I took the other two out.. He flares at any fish I put near his tank, so he only has snails for company :P They're about the only thing he tolerates.