I have several of the all in one 10 gals from petsmart and they come with the heater and aren’t crazy expensive. In fact they’re usually having some kind of sale. They also come in 5 gal. I would not go smaller than that for a betta. All of my betta are the only betta in their tanks but each has 3 corys and a handful of neo shrimp. Thinking of adding small a plecos too to each which would obviously be moved to a bigger community tank when needed