I have some tetras and barbs with my ellioti (similar to firemouth, though perhaps a little more aggressive) also a couple bristlenose. They have eaten 3 of the cherry barbs over the months, but I started with 8 and now 6 months later I still have 5. The tetras are red eye tetra and they are too big for them to bother and too quick I think too. My male elliota has to be approaching 5", the female 4". Reason I said another female is because with my pair the male was a bit rough with the female after she laid eggs, to the point I had to remove her. Perhaps with multiple females that aggression might be spread between them.
I never notice the ellioti chasing the dithers, oops I also have one swordtail. Swordtails do cohabitate with firemouths in the wild, they may work well. My one swordtail was a rescue from another of my tanks so hasn't been in for too long so I can't categorically claim they will be safe but I suspect if they are large enough (not juvies) they would be ok. Think green ones are the natives.
