I've kept a Betta in a tank with platys and catfish, and all got along just fine. I thought it was a really nice combo. My Betta would occasionally chase the platys if they got too close, but the tank was large enough that he had places to hide and he spent a lot of time harassing the little pond snails that found their way into my tank (he was great at keeping the population down).
I would consider a heater necessary for a Betta. In fact, a lot of the fish on the list are tropical and should have heat. In perfect conditions, bettas can live 9 or 10 years! Any I've kept before researching how to keep them only lived a max of 3. That's a pretty significant difference, and while I had those fish, they all "looked" healthy but in the end they died really early. Granted, it's difficult to know how old they are when you buy them but a few I knew were pretty young judging from the tail size.
I do agree with Stacy that gouramis are not a good choice. They are in a similar family as bettas and don't get along. I also don't think it is easy to put females together.