I agree that you are going to need a lot more of them... And more space.
I don't, however, agree about the temperature. They are more than happy all the way up to around 80. They breed like rabbits at around 76 to 78 so that is where most shops and breeders keep their tanks.
They are fine in low temps - but the water in their natural habitat does get a lot warmer in the summer. I think that a lot of places forget that there are huge differences in temp over the year for these guys (and goldfish).
The main reason I wouldn't want to keep them with a betta at that temperature is that they like to have seasonal fluctuations, not all year round warmth. Another reason is that they will eat anything, even pellets that look way too big. Mine would steal from my goldfish when I had them in the same tank for a bit. The goldfish pellets were huge, but they would choke them down. I have no doubt that they would dart in and steal from your betta.
Which reminds me - they are one of the only fish I would actually suggest to keep with a fancy goldfish. (Or rather, a goldfish too small or too slow to possibly eat them.) They will eat the same food and like the same water.