I have a 150g and have kept cardnal tetras, neon tetras, jack dempsey, electric blue jack dempsey, Blood parrots, guppies, oscars with my convicts. Not all at the same time of course, the oscars would eat most. I had some medium sized oscars with a dime sized jack dempsey and the oscars never even tried to hurt the jd or convicts.
right now my convicts have reproduced and I have about 4 pairs and many dime sized convicts, 2 blood parrots, a 5 inch JD, 1.5in electric blue JD, african knife and a few guppies. All those have been in there about 2 years or so. I have not lost a fish yet. I often have a bunch of convict fry in the tank, but most get eaten by the plecos before they get too big. Some make it to be larger though. I need to take some to the fish store so I don't get too many though. If I counted all the convicts even the small ones, I probley have 50 or so right now.
I do 50% water changes every week as well as a drip. I have never had a water quality problem so far. I have had a few sensitive fish in there such as the cardinals and the ebjd.
I am in the process of building a cave wall to give the convicts more tank area that they can use. I have one wall up right now and they love it.