Hello all-Firemouths

It sounds like you have what's a equivalent to 75 US Gallons (if that's a 4ft.x2ft foot print. If its more like 4ft. x 1ft x 2ft(water depth) then you have a 55 Gallon). Either way, with six FM's you are probably stocked as far cichlids go. I currently have 3FM's and 1 JD in a 55. (With some small cons as dithers that are getting to big). Two of the FM's seem to have paried off and my JD has grown a lot over ther year. I wouldn't want more than 2 pairs of FM's in a 55 gallon with dithers. So if you have a 75 6 fish COULD work, but If you are looking for cichlid variety you might consider keeping 1 or 2 pair (assuming you are working with a 75) and Maybe something like a single JD. I would suggest convicts, BUT there is the possibility that they will take over the entire tank.

If you want to keep all six pairs, I'd suggest going with a group of swords of mollies. They are pretty entertaining, can be colorful (i'd go with green swords to keep it natural), and they occur naturally in the same areas as FM's.

I love my FM's. I heard how aggressive they were, but they don't even pay attention to any of their other tankmates, they just get out of my JD's way. The only agression has been conspecific and has resulted in nipped fins, but that's it. They are mostly all bark and no bite. very entertaining to watch, and one day I hope to have a biotope setup like you, with just FM's and dithers.

I agree with AU Chief , 6 FMs in a 4ft tank is do-able but not really optimal. The big problem is going to be territory , and spawning. The territory thing can probably work its self out but when these guys start spawning it can get crazy. You could have a spawning pair defending their territory and chasing the intruder into another Fms space causing even more problems. Seeing as how FMs can be very nasty when guarding their brood it may be better to keep the two pair you like best and then return the others once the pairs form.
I also agree with a Dempsey for a tankmate, along with some suitable dithers like Giant Danios ( not bio-type correct, but hardy and fast enough to avoid trouble), Buenos Aires tetras ( look similar to a native tetra from that region ) and swordtails. At least untill the JD gets big enough to eat them.