I keep a severum in a 75 gallon. I've kept a breeding pair in a 75 gallon and it worked rather well but I wouldn't add other cichlids into a 75 with a pair. Right now I've got a male gold sev in a 75 with some growout central american heterospila and it's fine for now but eventually the severum will have to move.
If it were me, I'd consider doing a group of guianacara sp in a 75 with some tetra dithers, and some bottom dwelling catfish like pictus/hoplos/corydoras. If you think you'd rather go the central route, perhaps a growout group of sajicas with some livebearer dithers or tetra dithers. Eventually you'll probably end up with a male/female pair or perhaps 2 pair in a 75. If you wanted to try and combine multiple species of cichlids I'd probably try a Sajica and EB Acara as my first attempt.