With your almost neutral water you can do just about anything, except wild caught from the extremes of either end of the water parameter spectrum.
Because I'm a bit anal about geographical correctness, below would be my idea of how to go...
A South American biotope from west of the Andes would include your GT, festae, Geophagus steindachneri pellegrini and Mesoheros gephyrum. (below)
East of the Andes the mid country, pikes, most Geophagus, oscars, and the Heros species. Acarichthys below
Southern S America might include Gymnogeophagus, Australoheros, and some pike cichlids from the cooler waters of the south (Gymnogeophagus quilero below)
But....
If you went Central American, attention to aggression becomes the most critical factor.
Firemouths and the other Thorichthys, and Cribroheros are shoaling type earth eaters, and fairly non-aggressive, so other species tank mates (if any) must be chosen carefully.
Cribroheros rostratum below
Amphilophus are generally very aggressive but if grown up together as a cohesive group can work well, but preclude other less aggressive species (like FMs much of the time.)
To me, if looking at Parachromis, a 210 is only large enough for a pair of managuense or other Parachromis species, but no room for any other cichlids.
Of course if you start them small, growing anything together might work. But Starting them together all the same day, and then not adding others piece meal along the way, seems to work (at least for me)
Above is a non-geographically correct tank I had years ago with festae, Amphillophus amarillo, and Chuco intermedia (and a few others).
