rmcder;1255776; said:It's hard to say what will and will not work; fish vary as to their temperament. My experiences suggest that firemouth can't hang with anything that is genuinely tough, but are too stupid not to try. If your gt and dempsey continue to get along, then you've dodged one potential bullet. In that case, adding in the convict will most likely be fine. You might even try out a salvini female instead; maybe you'll get lucky. I just can't see a full-grown gt being happy in a 55g tank; add in a couple other good-sized fish and a bad situation gets worse. That's just me.
Otoh, I know people who subscribe to the overstocking to spread aggression theory, which, I guess, works with Africans. I'm not convinced it works with new world cichlids, but who knows... Either way, the 75g is a better choice than the 55g.

I'g love to see a 55 with a female Salvini and a Blue JD
But In your case I'd put the sal or pair of sals (whichever you choose) in the 55 and the GT and JD in the bigger tank. Would hold off on adding others for a while though.
Good luck sounds like a great set up to me.
Oh and as far as filtration, I'd look into the AC110 HOB filters. They're cheap, easy to clean, and quiet. I have 2 on my 90 and they're filled with media and are great filters