Agreed on rays... And a 180 will be fine for an ornate.
Maybe stick with the SA theme? Get some Geophagus or the likes. If the oscar isn't a fan maybe more plecs? Not too clued in on these fish so just an idea.
The 180 is too small for stock list and Tbh the bioload would be extremely hard to maintain. Most Ray keepers have their aquariums on a drip system. I also would suggest a sump over a canister filter. Welcome aboard
Oscars are cool fishes with some attitude. They are easy to keep when quality feeding, good filtration and space provided. From last 2years I am keeping and raising them from baby fish to large size.
You need to drastically rethink this maintainance strategy, even if you go with a slightly thinner stock than your proposed wish list. I'm betting that your 2x25% changes will lead to a very quick nitrate creep. Once a week fin level will be better, though nitrate testing for the first few weeks will tell you exactly where you need to be.