My single male Sajica has made an excellent community cichlid. He rarely shows aggression and if he does it's little more than a "shoo out of my bubble" sort of thing. Breeding is something completely different but without any partners I think he would be fine. My male (top left in my avatar) has been in with bichirs off and on for the last couple of years. No issues. I've never bread rainbows with bichirs but when I had them for a short time their breeding aggression was minimal. But others may have a better idea
At one point i had a polli, standard sen, albino sen, WC delhezi, CB delhezi, and 2 ropes in a 75. Not counting the ropes, the polli and standard sen was the most outgoing closely followed by the albino sen, WC del, and finally CB del. I rehomed both dels (I still regret that as I miss them) and the standard sen developed a blockage/tumor and wasted away to nothing and I had to euthenize.
If I got the pair of rainbows first would the pair potentially split up when the sajica is added? If it would be more trouble than needed I can just look into different cichlids. I’m totally new to them and dont have a whole lot of knowledge on the different spp out there.