I over-filtered my 10 gallon, had lots of ammonia uptaking plants such as milfoil and duckweed, and did water changes twice a week.
If you have some other species they can help distract your females from each other. If you went for a schooling fish I would be more inclined to use a robust danio or rasbora species. They tend to be fast enough and tough enough to handle any aggression the females might throw at them.
Peaceful algae eaters or lower level fish such as otocinclus, small plecos or corydoras can also work.
I think it just depends on the temperament of your sorority as a whole. I couldn't put any fish in with my sorority. They killed and ate two of the medaka I was using as test fish in about the first half hour of introduction so you do have to keep that in mind. I would probably actually purchase the other fish first and then introduce your females into the sorority.
I've found bettas tend to react with less aggression when they are the intruding on another fish's territory.