I have kept a group of girls in a 10 gallon tank for two years now and they all get along just great. I started with a group of 7 that I purchased as babies from a communal tank. They have never squabbled after the first few days of establishing their pecking order. Even then, it was just a bit of chasing (not nearly as bad as danios or tetras lol). That group is down to 5 now. I lost two of them in the last month to old age
I also have another group of 4 females in a 20 gallon tank and they get along great, too. These fish were not purchased in cups, either, but from a communal tank.
In my female betta tanks, I provide lots of caves and plants. You can have a very pretty tank, because the females now have bright colours, comparable to the males

I would recommend starting with at least 5-6 females in a 10 gallon tank. In my 20, I only have 4 because that was all that was available at the time that I bought them. It is best to add them all at once.