I would not keep Bettas with livebearers, beacause they may confuse them as rival Bettas due to their colors and finnage and attack them, and especially not with barbs because they tend to get nippy. If you want a Betta, I suggest setting up a separate 5 gallon tank just for him.
I've kept Bettas with Platys, mollys and Corys. But the Betta will eat any fry that they have. The Betta- GouramI combination is asking for trouble. They are both aggressive and will tend to fight. Don't know about the Barbs, never tried that combo. You need to consider the size of your tank before buying moe fish