a "big" one means different things to different people. some here would consider a 40 gallon tank "big". i breed cichlids and consider anything under 75 on the small side. 'big' to me means 250 or more gallons.
a flowerhorn would probably dismantle a blood parrot or severum even in a 125 gallon. Better tankmates for severums would be other South American cichlids like chocolate cichlds or some of the larger acaras.