I think sometimes you just get lucky with fish that in some circumstances can be very aggressive. Fire mouth cichlids aren't particularly aggressive but they do have their moments just like Severums for instance, these fish are not known to be aggressive but again, they can be aggressive on occasions, especially when they got eggs. These fish may be smaller than Oscars but don't be fooled by their size, they will stand their ground.
No, the Oscar won't refuse food just because there is another fish in there with him. Oscars can be very funny fish, sometimes I put food in, one of my Oscars will swim off as if he is in some kind of mood, it's quite weird I really don't know why they do it. And then when all the other fish have eaten the food, he will decide to come back when there's nothing left, makes me feel really sorry for him, of course than I put a couple of pellets in for him so at least he's got something to eat.
There are lots of different types of pellets available, Oscars loves shellfish such as cockles and mussels. You can feed them squid, fish, you can even mince it up and put it in a little block with gelatine so it sticks together, they love this also. Go to your local fish store and have a look in their freezer, they normally have a large variety of food that is suitable for Oscars.