Having had 11 full-grown oscars in a 240 gallon tank at one point (WAY to many btw!), my first recommendation is DON'T OVERFEED! O's ALWAYS act hungry, but in reality feeding them 2-3 times a week is sufficient. Also, I find that feeding them frozen/fresh tilapia or shrimp produces FAR LESS waste than feeding them any kind of pellets. Pellets = mess, at least as far as O's are concerned!
I noticed you had gold fishes in there as feeders presumably, while it's fun to watch the oscar's chase I would HIGHLY recommend you keep them in a separate tank and only put in as many as the O's will eat at a given time. It may seem more 'natural' to keep the gold fish in there, but they will certainly do their part in dirtying up your tank!
Be careful of pleco's...having too many pleco's will actually increase the amount of debris on the floor and may even encourage them to switch from algae to other foods since there's more competition.
I like your idea of putting a powerhead or other pump near the bottom to keep it from stagnating, I've done that in some of my larger tanks and it particularly helps with my big catfish and pleco's so that the overflows have a better chance of picking it up & adding oxygen/water flow.
But as others have said, with O's the reality is you have to keep up on maintenance. Good luck!