It's already been mentioned but severums make great tank mates. I have kept my pair with at different times but sevs, geophagus, satanoperca, large crenicichla (9" female sax pike), juvie orange heads. Tank size was 220 and 300 gallons. I don't like keeping them in tanks smaller that a 6'x2' footprint because of adult size. My female is the most dominent in the 300 gallon. She rules her tank.
Getting them to get along with tank mates is easy, if you want "extra credit" get them to be tankmates with plants...