In 6 gallons, there isn't enough room for the fish to get away from each other, so your betta can be stressed. They honestly don't enjoy too much activity via the other fish.
2 gallons is the minimum, and I'm pretty sure the medium sized critterkeepers are pretty much that. He might enjoy being just by himself in there, some bettas aren't cut out for fast paced tank mates. Of course, in bowls, bigger is always better. Betta's can live in those smaller bowls, but like alex said, good temps, and clean water are important. The only problem I have with small tanks is that my bettas tend to get bored and start to fin nip. So long as you keep him healthy and stimulated, he should be fine.