A Datnoid would be a nice addition but ive kept my oscar with my ornate for awhile now and they get along fine! The Oscar tells him whose boss now and again but there usually fine even at feeding time?!
For something different Id say a species of gymnogeophagus! They are earth eaters that dont get too big but big enough not to be eaten and they are peaceful! Or maybe a Leopard Ctenopoma. I have both above species with my SAL (S.A. Lungfish) and there have been no problems!
I do the poly/severum combination, and I like it. The bichirs spend most of their time at the bottom of the tank, and the severum are all over the place.