When setting up a tank I like to go kinda small for the simple reason that it makes everything else look larger in perspective. There are many cichlids that would work well in a tank that size; being the focal point but not taking away from your options in the future. A pair of Severums would make a nice centerpiece while still allowing you to keep Rams or Apistos.
There is a vast list of suitably sized cichlids for a 90; it really depends on what you want in the tank. Multiple species? Single species? Plants? You're blessed with water that will support a thriving planted tank. I'd recommend you look into the profiles for south american cichlids at cichlid forum and pick the fish you MUST have. Then plan your stock around that species: http://www.cichlid-forum.com/profiles/results.php
There is a vast list of suitably sized cichlids for a 90; it really depends on what you want in the tank. Multiple species? Single species? Plants? You're blessed with water that will support a thriving planted tank. I'd recommend you look into the profiles for south american cichlids at cichlid forum and pick the fish you MUST have. Then plan your stock around that species: http://www.cichlid-forum.com/profiles/results.php
I keep Geophagus sp. 'Tapajos' orange heads and they are wonderful fish with personality that gets better the more you have. Like Abarilot suggested and you've researched, Altifrons get too large for your tank. Have you considered a school of rummynose tetras? They are not region specific but look great together IMO. I have been wanting to add a school to my OH group of 9 along with Sterbai Corydoras.
