Everything should be okay in that size tank if you raise them up together.
Oscars aren't really aggressive and neither are severums, if they end up fighting just divide your tank in half. A severum could never hold it's own against an angry oscar.
Should be fine. I've had that combo many times with no problems. If you do the grow them out together route best bet is to start with your sev(s) bigger than your O, as the O will grow faster.
Deinitely can do. I had One 9-10" Oscar(just traded him in today) in with my pair of severums with no problems. The severums would push the Oscar around when they bred. I agree with the others, get the severums bigger than the oscar, or grow the severums out some before adding the oscar.