Lots of plants & hiding places. I currently have 13 in a 46g tank with no real fighting going on just some chasing here & there. They're only tankmates are 2 CAE's & some snails. Im not sure how your other fish would get along with them since I don't have that kind of experience.
You may want to add a few more tetras. They are happiest in groups of 5 or more. If you do the Bettas get them all at once and put them all in at once. Its better to get sisters who have been raised together.
Thanks. I have a decorative log in there, along with a amazon swordtail plant. My LFS has a female betta tank, and I want to do it. They have really pretty girls.
Question: should they all be the same size, or does it matter? And how many in my 55?