I'm not entirely sure, but I'd say you could fit four bettas in a six gallon. I don't recall the thread, but Wuv said something along the lines of it's better to overstock a sorority tank than understock and mentioned the maximum she'd put in a ten gallon is 8 girls. 8/10 is more overstocked than 4/6 (percentage wise atleast). Just be sure to give them lots of plants and hiding spots so that in the small space they do not get to aggressive with each other.
When I got my girls, they also were rather down looking at the LFS in their little cups. They really did perk up after having room to swim, live plants and some good eatin' (I didn't even add any salt... but I have cories too). Even my mother (who doesn't really care about fish) mentioned to me how different they looked after just a few days.