In my 210g I have a rhino pleco in with my Del and weeksii and they do ok. The pleco will on a rare occasion attack a bichir as a territorial thing and that is all Ive seen. The trick is to keep them well fed and have lots of hiding places.
I think 4 oscars are fine I did Oscar tanks for years, one of the most fail safe fish iMO. I finally stopped keeping them becuz its impossible to keep a nice tank when they get larger they will knock over every thing they can and move a lot of gravel, I just ran out of energy but they are awesome fish, just watch the gold fish they are super prone to hole in the head.
Update: So I came across this really good looking albino oscar and went back to my original plan for my 125 of one black one white yen/yang oscar tank. New stock is two oscar and two bichir.
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