Many people keep sorority tanks, with all females. Females can still be aggressive and even kill each other. It’s important to make sure the tank is big enough. In the wild, betta rarely kill each other in these territorial disputes or fights over females as the weaker fish has room to run and hide from the dominant, stronger individual. In a fish tank, especially a small one with little to no decor there is no where for the weaker fish to hide, so the stronger/dominant individual will harass and attack the weaker fish until its dead. The best way to keep females together is provide a large tank with a lot of decor or a heavily planted tank. Its important to provide decor/plants that divide the tank into sections and break the lines of sight between fish, and hollowed out decor for fish to hide in. Males can and will kill a female if she can’t get away from him and I’ve had females seriously rough up a male that wouldn't leave them alone too.