FreshyFresh (Joel) has kept an oscar & a severum together for quite a long time. I bet he chimes in soon. Yes, you can keep just 1.
Any fish that has been line bred for a genetic variation is likely to not live as long as a "normal" fish. All that selective inbreeding usually leads to weaker (shorter lived) fish. But it also depends on good care; lots of clean water & good foods
