What Heath says is true, but there is unfortunately more to this. And I see a problem here.
I always counsel against cichids with Tiger Barbs or similar fish that tend to fin nip. The temptation is too great. Cichlids are all fairly sedate fish; they rarely swim much (in the sense that barbs are active swimmers), preferring to cruise around poking at the substrate, bogwood, plant leaves for tidbits of food. Most cichlids have fins that frequently wave a bit. All of this is like the red flag in front of a bull. The barbs like the bull may not attack, but experience shows they often will.
The other thing is that once the barbs have begun nipping, they rarely stop no matter what changes. Adding more will not hurt, but don't expect the nipping to cease. It may increase or stay the same.
As for the Firemouth, although this is a mildly aggressive cichlid, it is also a very nervous fish; external noise will stress it, and tankmates must not be aggressive or boisterous. An aquarium with plenty of branches and floating plants will suit it admirably. It prefers quiet water, so the filter flow should not be excessive. The nipping of the barbs may well "destroy" this fish if not quickly resolved; by destroy I do not mean kill it, but this sort of bullying usually if not always causes severe stress which when it becomes acute as it quickly does, means permanent internal damage and a weakened fish.
I would recommend you consider which of two options you have. First is to stay with your intended cichlids, in which case I would re-home the Tiger Barb now and find more suitable fish. There are some nice barbs that would work well...one that comes to mind is the Black Ruby; it has a somewhat similar pattern, plus that beautiful ruby red head and almost totally black of the mature males. I have a group of these in with Congo Tetra, also flowing fins, without any problems, and I have had this combo for three years. I recently added more young barbs (Black Rubys) and they too have not even looked at the Congo's fins. You have the space and could increase the Firemouths; they do well in a small groups. In its natural habitat this species is always found in such groups, though not strictly a "shoaling" fish like the barbs; nervous fish do find security with others around them, and this is no doubt the case here.
Second option is to stay with the Tigers, and up the group, but remove the Firemouth and forget cichlids. Other barbs, danios, substrate fish are possible.
Hope this helps.
Byron.
