That's way too many fish for that tank for one. You should return the oscars as even one oscar needs about 100 gallons to itself. After you get rid of the oscars and the red devil the aggression should start to subside. However, unless you pretty much return all the fish, except the pleco the aggression will never go away.
Bottom line, you can't really mix large predatory SA cichlids unless you have a very large tank and sorry to say that 75 gallons is nowhere near how big of a tank you'll need.
Cdawson, you think my tank is overstocked, look at what Petsmart recommends. Yesterday I stopped at Petsmart and noticed they have a recommended compatability/stocking chart according to the various size tanks and type of fish. For my 75 gallon tank they suggest stocking the South American Cichlid tank with a total of 11 fishes: 2 Oscars, 2 Jack Dempseys, 2 Green Severums, 2 Firemouths, 2 Convicts, and 1 Plecostomus. Now thats a lot of Fish!