I have 8 females + 3 other fish in my 15g bow front and my females never fight, I think your biggest problem is the lack of plants/hiding places in your tank. My tank is very heavily planted, and I leave a lot of plants floating, when I first started my 15g sorority I even left big amazon swords floating for more cover(I planted them a few weeks later so they wouldn't die). I was also using almond leaves(blackwater extract also works) so that my girls stayed calmer and didn't stress about the new surroundings. Sorry about the algea on the walls that make my floating plants really hard to see.
My aquarium is over stocked as well, but I do extra water changes to counter this, and the stem plants also help.