I am not too sure but my gut feeling says that your stocking is already close to the limit,
depending on the species that you haven't listed the exact amounts.
If you however are able to keep your nitrates consistently below 20 ppm, some way or the other, then
I would recommend looking into a peaceful larger centerpiece fish, as you already have lots of schoolers.
peaceful gouramis or dwarf cichlids such as rams are the first things that come to mind for me. maybe put your apistogramma
cacatuoides in the main tank and see how it works out?