Well - for the record, Oscars get on very well with blood parrots and pl*cos in my experience, but unless you get rid of everything else in there you'd be too crowded.
As you have a balanced population as is, if it aint broke - don't fix it.
Two 304's would be fine btw - Oscars are heavy hitters (left out an 's' there) but they're not that bad.
Oscars don't go with everthing just because of size - most times it'll end in a death match - check first.