Also, I think 18 inches for an Oscar would be a really huge, rare Oscar. A 75 would be a great home for him, by himself. If you want other fish, too, then start looking at six-foot tanks -- in terms of bioload, territory issues, etc.
Butterfly with 2 syno's is fine.
I do hope you actually remove the oscar like you say you will, and the dat!! Thats 2 fish that will hit a foot long, in a 2 1/2 foot long tank!