Um dude, your tank is overstocked majorly. Your Firemouths are going to get a lot bigger and will need much much much more room (as in a 40-50 gallon tank). Unless you are planning to upgrade in the near future, you shouldnt be adding any more fish, you should be getting rid of some...
My convicts always fought with my firemouths because they are very territorial. I had a jeweled chiclid that got along fine and the firemouths swam with them all the time and followed each other around.
In my own limited experience, firemouth aggression is more intraspecific (towards it's own/similar species) than interspecific (other species). This looks like a dominant fish beating the tar out of subordinate fish.
That being said, they can be aggressive, but not nearly as much as other South/Central American cichlids. You may want to try the smallest geophagus, blue acara etc.
You may want other medium central american cichlids, but as for someone saying put a "such and such" in it's a guess at best, depending upon a bunch of factors (size sex, "personality", water temp etc.). When I was a teenager, I had firemouths with fast moving fish like tiger barbs and found they had didn't bug eachother at all. I know that diverges from a pure cichlid theme. With stocking cichlids, other than giving them a lot of room, I have heard of many strange combinations working.
Just my ramblings and probably putting you back where you started.
Im glad to see you took the advice some of the people on this forum are just so ignorant. Anyways, I would probably stock it with a couple nice severums. They have some really nice vivdly colored ones (Bright red with orange specles) at King Eds Pet Center on Kingsway. I would stay away from anymore firemouths unless you are going to have 5-6 of them in the tank to spread out the agression (but then it would be really squished space-wise). So, maybe one red severum, and one green severum or even a few Kribensis Cichlids, they are strong, and very colorful and stay quiete small. Great fun to watch and will probably have babies.
Just read the other post, above me. I would say no geophagus. I had 6 geophagus heckelii and red head tapajos mixes and then I added a firemouth and it started nipping. Barbs can nip too if there arent alot of them to spread agression and loaches tend to get sick fast (I keep valued fish and care for them well
I've kept red-humped eartheaters (G. Steindachneri) with JD's and Severums (the tanks were 90 & 135 gallins though).
It was actually a good mix.
The Steinie is one of the rowdier geos so they can hang with some of the more docile CA's - like firemouths.
Another good thing is that as mouthbrooders Steinies don't get territorial like the CA cichlids will if they breed. Bad news is that you should have two females for every male and the females can get to 5 or 6 inches (I've seen big males in the 7+" range, but 6" is more common). So a 40g tank may be too small for a trio + FM's.
You never mentioned - or at least I never saw it, are your Firemouths a pair? If they are you may not be able to keep much else with them in the 40g (other than the pleco) as they can be kind of grumpy when they spawn.