Severums are one of the most placid and peacefull south american cichlids they do well with smaller cichlids and larger peacefull cichlids, they grow to a size of 8 to 10" people say they have them larger but 8" is there average size in the home aquarium.
Tankmates i'd have in a 75gallon would be 1 other severum a female if possible ( incase you didn't know how to sex them it's so easy the male will have a dark pattern on his face he'll also have longer anal and dorsal fins ), anyway they do well with most other cichlids such as convicts, firemouths, jack dempsey's, slavini's, geophagus even oscars i have 3x 12" oscars and a 8" green severum and i have no problems.
I would think by you adding some clown loaches that would be fine you shouldn't have any problems with your severums wanting to eat them as long as there not 1" long because severums have quite small mouths which pretty much limits there preyso if you get a few 3" clown loaches in there shouldn't be a problem at all.
Dan.