As noted in that response of mine that you quoted, look at barbs and danios, and also some of the tetra (characins) can work. Of course, livebearers are another option (as they come from the same region, Mexico) if you have moderately hard water.
I don't usually like suggesting specific fish, as you might end up having "my" aquarium instead of "your" aquarium. When species that are likely to cause trouble are mentioned, I comment. And here I suggested the Black Ruby Barbs only because they do bear a striking similarity to the Tiger Barb but do not carry the nipping. However, with the lone Firemouth, companions will have to be carefully selected...very carefully. As I may have mentioned previously, Firemouths are somewhat timid, and as this is a species that does occur in loose groups, a single fish is going to feel very much "alone." And that means tankmates must not be at all intimidating, or the poor Firemouth will be stressed and that quickly leads to poor health, or increased aggression.
With a 90g, you could have a small group...have you considered that? Then the "dither fish" would perhaps be easier. And the Firemouths would likely be less inclined to be a pest, although spawning would be an issue obviously. I only mention this as I do not know how attached you are to cichlids. I am a dwarf cichlid person when it comes to this group of fish, and after several community setups with various dwarfs, it is not something I find easy to handle. Other fish can get picked on during spawning, and sometimes seriously, even by rather small female apistogramma that can turn more vicious than many would expect.
Byron.
