Females are hard to get along in a ten gallon. You do need a minimum of 5 though, for any tank size. this helps spread out aggression, just two or three, they will shread or kill one another, four is do able, but expect hell for the first week or two as they are setting up their pecking order.
I personally have 10 or so females in a 55, they have been there for a few months, not one casualty.
Males need a lot more space to be compatilble, and once again, it all comes down to temperment. I would say it toke me a month to find the right combination of male bettas before I could have them in a 55 with out problems. I can tell you this though, NO RED BETTAS, they will not get along, and they will be a number one target for most of the other bettas. when they said the bulls attack because of the color red, they should have said bettas.