female bettas should usually be kept in groups of 3 or 4 to prevent/spread out aggression. The recommended minimum size tank for a group of 4 females is a 10G..if you don't have any other fish in teh 6G, you might be able to fit in 3 or 4, but they might be a bit aggressive. if you really wanted to do it i'd keep the tank very well planted, but also have some kind of a backup tank for the bettas in case they do get too aggressive.