with a 75g if you want more than 2 fish then i would stay away from the gt or anything else that gets 10in, cons n firemouths are fine but i personally like something more colorful, like pyramid suggested sajicas n jewels are nice you could have a pair of both in a 75 with dithers and territories on opposite sides of tank with sight breaks and MAYBE even a single salvini, dont really know about blue acaras so cant say anything about them, salvinis are very cool fish but very fiesty so not sure what you could have with it, maybe if didnt work with the jewels you could do a sajica pair and 1 salvini with some kind of dither and a pictus would be a nice catfish that wouldnt get too big and IME is bossy enough to hold is own and is pretty much ignored