Join Date: Nov 2011
Location: SF East Bay Area
GBRs are cichlids, with all the good and bad that it entails. They are pretty sociable fish and tend to hang at the lower 1/3 of the tank, right where your hard-to-find C. hastatus should be. With most C. hastaus still being wild-caught, I did not find them the easiest fish to keep in a comunity tank. You do not mention the tank size, but I assume that a single GBR will be bored to death and make itself a nuisance to the cories. Will it go after Amanos would be my other concern.
Dwarf Gouramies can get pretty nasty, especially as it will be the largest fish and a male to boot. They do well solo and yours will most likely spend the time sleeping in the upper branches. They are also a lot more tolerant to water conditions and smaller spaces. Therefore, it would be my personal choice if you have to have one of the two.