Keeping nitrates below 10? What are you keeping, a reef tank?
Oscars and sevs are hardy fish. The only reason you want to keep up on waterchanges is to prevent Hole in the head in the oscar and the sev. Even then, a 30% waterchange using aged water is enough to prevent HITH.
You should be able to get away with a 3rd med sized fish either a med. sized cichlid, or a catfish/pleco. There was a member a while a go that was keeping an Oscar, Male Convict, and Male Salvini in a 75 gal for a while. A 3rd fish will break up the aggression between your Oscar and the Sev. If you don't go the catfish/pleco route, convicts make great dithers and won't add a lot to your bioload allready. They only get 5-6 inches in length and are able to take a beating. They come in either normal, pink, or calico.