It's possible no other fish will be ok depending on the temperament of the betta. I hear often that african dwarf frogs might be ok and are the best bet but bettas like to be alone.
That being said I see them in 10 gallon community tanks a lot at the fish stores, with other small fish that don't nip fins and don't look at all like a betta (no guppies, barbs, angels, etc).
I wouldn't go smaller than 10 if it isn't alone. 5 if it's alone. I have mine in a 3 and he does well but I'm aggressive about water quality and I've lost a few. I'm confident he's ok yet I hesitate to recommend the 3 unless you're experienced, do a ton of testing, and have learned a lot about the nitrogen cycle, pH, KH, GH, and tds.
The whisper internal filters are great for a betta because they have a very gentle flow.
The mini bow tanks are awful because the filter is too strong, it's too hard to get a heater in there, and it's difficult to get the lid on and off for water changes. Also the filter blocks the light.
I really like the Marineland/tetra led kits in 3 and 5 gallons, with a heater that had a thermostat.
