Like most fish, I found 2 in a tank does not work. 3 or more is best. I find all species together is no problem. I have 16 (11 sivers, 2 formosa and 3 jardini) together with no problems in my 15k
I've got three that I brought home together at about 5" - 6" that showed a little aggression towards one another maybe for the first n second week. But it's been alllll good from then on. Maybe keeping them well fed, and giving enough room will lessen the agression?