The tank and filtration sound's fine to me for the fish mentioned.
For tankmate(s) ,I might consider an Electric blue Dempsey that grow's large enough not to be viewed as food by the Oscar(assuming Oscar purchased is juvenile), and isn't nearly as aggressive as regular Blue/Green dempsey's.
Or I might consider a shoal of five or six, Synodontis Multipunctatus (grow to around seven inches).
Large driftwood pieces,sand substrtae(cheap), and smooth river stones,Flagstone, to make cave arrangement for the Synodontis.
Lot's of other not too aggressive larger South american cichlid's could work but i confess to being partial with regard's to the electric blue Dempsey's.
Kept one from two inches to adult size before I sold him to a couple who placed him in much larger tank than the 80 gal I kept him in .