Yeah, it's a shame these fish can't seem to get along with each other, because they are such a lovely fish...it would be very cool to have a tank full of them all different varieties...unfortunately, not possible to do and keep them all healthy and alive.
The best I have been able to manage is a 10 gallon divided setup. I have 4 males in a 10 gallon with plastic canvas dividers. It's a pretty tank and I love to watch them. They don't constantly war with each other through the divider either. They occasionally make faces at one another, but not excessively.
I have a mini penguin bio-wheel, airstone and heater in this tank...with some plastic canvas in front of the filter outlet to slow the current down for the betta in that section. I also have a piece of the plastic canvas at the top of the airstone, for the same reason. The bubbles collect under the plastic and don't stir up the water as much.
I also have a 10 gallon with 4 female bettas in it. They get along very well though occasionally I'll see fin damage from a tussle from time to time. I did have 5 in there but had to take one little girl out because she wouldn't defend herself in the initial "lets see if we can all get along" introduction. She has a place in my newt tank which she seems to enjoy. So, females can get along, but will occasionally do damage to each other. I have a very pretty yellow female which I keep by herself because I don't want her to get chewed up at all...she's a beauty and I'd like to keep her that way..lol.
So...the short of it is....yes, you can keep female bettas together, though they will suffer occasional damage and you do have to watch them at first to see who does well and who doesn't...and take the ones who don't ..out..
Males you can NOT have together unless there is something between them to keep them from eating each other alive...and even then you have to be sure it isn't something they can't jump over and get at each other. They are fantastic jumpers ! If you have an overly aggressive male you may not even be able to keep him next to another male or he will tear his own fins up just flashing around trying to get at the other guy.
Whew...long post, time to stop...lol....I could talk about these fish all day long...
