I've generally found the apistos to be pretty delicate. The hardiest one by far is the cockatoo cichlid, you could probably go with 2 males and 4 females in a tank that size, obviously without the fm's. Best in a 2:1 ratio you see, but the females are a bit drab. Talking about boring fish, I actually find the Discus to be pretty boring once you get past the lovely colours because they do very little. They just tend to hover together. They are more of an 'ornamental' fish than a pet. The firemouths are ok, but not as colourful as a lot of the similar sized / temperament.
I would strongly recommend the cryptoheros family, convict, Sajica, Panamensis etc. they are small growing, entertaining and quite colourful. The problem is if you get breeding pairs it will all kick off. Maybe go for one sex of different types. The Rainbow cichlids are colourful and not too aggressive.
If it was my tank, I'd probably keep your favourite Firemouth, and add a female con for blue, a Rainbow for yellow and if you could get a Panamensis one of those for a bit of red. You could perhaps house 5 cichlids of a sensible range of sizes 3-6" in that tank. If you've never been to Maidenhead Aquatics before I strongly recommend it. Phone the different ones in your area and see who has the best range in terms of your taste. Good luck anyway.
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