I also have a school of 6 red eye tetras, 3 cories, and usually 3-6 ghost shrimp in a sparsely planted aquarium and it is a peaceful environment. If you're worried about aggression, cories would be perfect because 1 they are tough and 2 they are mostly bottom dwellers. I would recommend pandas cories as I believe i saw an underwater vid of the Negra river and saw red eye tetras and what I think were panda cories...who knows, they get along great for me. And with the size tank you have, you could get a nice little shoal of cories, all the same kind, and they will stick together like a pack of little puppies.