The outcome of fish when breeding (i have no experience of breeding oscars) depends on many factors. There is no guarantee that when you breed 2 fish of the same sort that you get that sort as babies. Same goes for a mix of varieties, you can get some babies like one parent, some like the other(if neither colour is dominant) and some in between and some may be nothing like either. I think that when you get 2 fish breeding and all the babies look like the parents they are called a true pair. Its a bit like people and height, 2 medium height people have a kid and sometimes they have a tall(or short) kid, they dont always have a medium height kid
I read somewhere in a book about discus that they fix a new variety by breeding a offspring that looks like a parent with that parent(not sure on this tho)
When it comes to mixing the fish its all about mouth size. Your bigger oscars will eat the little ones if they will fit in their mouths. So the thing you should be looking at is how quick your little ones grow compared to how quick the bigger ones mouths grow. It is obvious that the little ones will grow quicker that the bigger ones mouths. A full size oscar couldnt eat a 5 inch oscar so you will not have to wait long. So just feed as normal and wait a while. When you add them tho keep an eye out for bullying.