Yeah 75 usg would be preferably, they tend to reach 8" but they also fill out alot, so they are very stocky fish.
Tankmates in a 75g would include some shoaling fish, obviously large enough not to fit in the Sev's mouth. Larger size tetras, rainbows etc.
If you wanted to add more cichlids, then 100g tank would be preferably, then you can mix them with acaras, angels, geo's etc, nothing overly aggressive though like a JD or GT, they can be in tanks (125g upwards) with an Oscar.
Fairly easy to care for, providing you a fully cycled tank, they are not too fussy on water parameters, weekly w/c are obviously needed, will eat pretty much anything. Tank temp between 24 - 28c.
Take a look at the Severum Appreciation Society pinned topic at the top of the NW cichlids forums (or there is a link in my sig) for links to some good information on Severums
