Some betta sororitys work. I have a 10 gallons with 5 peaceful females.
In my 36 gallon the two females just recently started fighting, after being together for over a year.
For how long?
Things work till they don't. I had 2 male golden wonder killis together for well over a year as mature specimens. One day the 125 wasn't big enough for the two of them. It's stuff like that that keeps us on our toes ;-)