Nijjensi are pair bonding so adding additional female would not be advisable. I could add 2 pairs but i think that would be a mistake. I could add some other docile dwarf cichild or just leave the rest of the tank empty or empty of cichlid. What I'll likely do is try 2 species that require low ph and if there are issues move one set to the basement and while the ph will not necessarily be sufficient for breeding it will at least be low tds; though the basement group will get little attention beyond clean water. I use the term breeding here in a weak sense. if there is sufficient coverage and space for the parents to raise the fry I will provide some food suitable for the fry but otherwise leave it to the parents to raise them to around a 1/4 inch after which I would move them to a grow out aquarium until i can give them away. I don't sell fishes but for less common species i will donate to whomever wants them or as a last resort give them to a fellow i know who has a store.I have mixed species in the past. I have never found they complimented each other. If I had room for two species, the tank was more enjoyable if I just upped the number of females of a single species.
I was mixing in a 6’ tank.
I wont mix species anymore. Unless its a grow out situation, but would still prefer to keep them apart.