Hemichromis species are from the rivers of africa, so I would not suggest a jewel.
Normally I would heavily suggest a severum and blue acaras..I own both, and find the sevs to be my favorite of all my fish. I'm not sure how they would do with convicts though... I have a breeding pair of cons, and i think they are pretty mean.
Jack Dempsey may or may not be a good choice. I love the fish, and I've owned two. One was a lethargic psycho which cut a convict in half....but this was in a smaller tank. My current dempsey bothers noone, and is always out. Eats out of my hand readily, and even follows me across the tank....I think that has to do with the fact I bought it young, raised it in a 150 gallon tank, and has many hiding spots as well as dither fish.
Anyways, I'd ask around and get second opinions from other members about compatibility, as my experience is fairly limited.
Some fish I would consider though are:
Firemouths
Severums ( if the cons won't tear them up )
Blue Acara ( Same deal )
Jack Dempsey
Chocolate cichlid ( not sure how it would work out, though )
and I suppose there are alot of other fish as well.
I think if you had alot of hiding spots in a large tank ( 75 gallons isn't bad! ) with relatively few cichlids and dither fish, AND you purchased other fish larger than the cons, you could have some pretty good luck. That way the cons wouldn't be as likely to attach a larger fish, yet they could still stake out territory. The dithers would distract them as well, and hiding spots help everyone.
Good luck...