If you're gonna go with cories, the Oscar will eat them when it gets big. I would chose a big Raphael or a synodontis eupertus. The syno will hold its own against an Oscar. Both stay under 10". Or if you want an active cat that stays small I would pick a Pimelodus blochii, maxes around 10".
If your lucky you might be ablt to have the cory's with the oscars. I used to keep tiny 1 - 2 inch baby clown loaches with my 14in Oscar with no problems. Just depends on your Oscars personality
If you want an active cat that you will see alot get the synodontis. I have kept them with many oscars over the years and my biggest one topped out around 9''. Corys are great but the oscars will eventually be big enough to eat them and if they did could kill the catfish and the oscar as well. Raphaels are OK but usually once you put them in the tank you never see them again, unless you have a bare tank where they cant hide. What ever you choose looks like you are def on the right track and doing things right
