If you don't care about being geographically correct, then don't read on.
I however am a bit anal about it, and always see tanks of mixed continent fish to appear articial, and at times even goofy.
Firemouths are from Mexico, and share waters with livebearers, and Astyanax tetras.
Barbs are either Asian or African (Odessas come from Myanmar (Burma), and to me would never be found with fire mouths.
Same with jewels being African, beside being uber aggressive compared to FMs.
Even Apistogramma seem out of place with FMs being from no where near Mexico, and are generally considered to be a soft water genus, unlike the Thorichthys which are generally hard water fish.
If you are dead set on barbs, the Asian cichlids Etroplus canaraensus or maculatus would be more apropos, at least coming from the same continent, and these cichlids would actually fit in your size tank.
I believe a proper size group of FMs will quickly out grow your tank. I have seen 10-12" males.
canarensus below.
