I, personally would have at least a 75 gallon with very good filtration. Oscars are very messy fish in general. As for feeding, in my experience, if you start them off with oscar or cichlids pellets, which come in different sizes for the smaller as opposed to larger fish, they will have a better diet than feeding feeders to them. You can also feed treats of bloodworms, brine shrimp, etc. I have never used things like beefheart and other animal byproducts as they can be very messy in the tank. Its been my experience that they will eat when they mature, the same as they have eaten in their younger lifetime. If you introduce feeders as food, then yes, they are going to get accustomed to them and want nothing to do with the other food. Feeders can introduce illness into a tank, how healthy can they really be living with 200 others in a 20 gallon (if that) tank. Surely they can't all get ample food to stay healthy and the water conditions can't be good. Personally I would refrain from ever introducing feeders as oscars like us, need a balanced diet to maintain optimum health.
Allie