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.