Tankmates?

The firemouths and rainbows would mix well enough, and you could also keep a school of tetras, catfish, etc. for variety.
The JD would limit your options quite a bit more. You could keep other mid-size cichlids of similar (aggressive) temprament, maybe a catfish or two.

I have a large oscar, albino chocolate pleco, and small male convict in my 90g with my female jack dempsey. What she lacks in size, she makes up for in her temperment. Beautiful she is, but only gives the oscar respect becuase he is nearly twice her size. Although, I have seen her grab ahold of his stomach once during feeding time, creating a nice wound on the oscar's underside. Good luck in whatever you choose.

I've found that chiclids that grow up together genrally tolerate each other much more than adding fish from other sources when older. This creates friction as they battle for the alpha position, as they are larger when older they do more damage to each other. As stocking a 90g I would have some geophagus surnimensis great fish. Pretty much zero attitude and a realyy friendly temprament, although there dull coloring when young they start to bloom at about 8 months to 1 year. Seriously these are the beat chiclids I've kept for over ten years

salvinis

The three in your list would work together nicely.