When it comes to males, it is definitely not recommended to keep them together. They will either kill each other right away for territory, or they will lead very stressed out lives.
Females on the other hand are totally capable of co-existing. They can be very aggressive, but if you keep them in groups of 4 or more, the aggression is spread out and they should be able to live together in a minimum of 10 gallons, with lots of plants and hiding places. I have a female betta fish sorority myself. They fought like hell for maybe the first 24 hours, now they're all for the most part peaceful with eachother... except for Daiquiri... my little dictator... had to remove her for a bit just because she was just that aggressive. Drama. lol... I could probably link you to a couple of helpful forums if you were thinking of starting one up if you're interested!
Winston~ Damian~ Twitch ~ Evans