A Betta? It's not a problem really, provided it's either filtered or gets 100% water change frequently, and it's large enough. At least a gallon. If he seems unhappy, why not try to get a 10 gallon kit or something similar? They can be pretty cheap, and you could even divide it and get 2-3 Bettas if you wanted.
There has only been one type of betta that I have ever kept with livebearers and that is a female, they were just fine together too because of the females decreased agression
Not all females have decreased aggression, some can be as bad as males. It depends on the fish, and the tank size and Livebearer too. Either way, it's a pretty risky buisness.
it is very normal if they don't have colour. Mine didn't show up to about 3 - 4 weeks. Sometimes it just takes time and patience but they will eventually get their colour!