Depends on how you run it. By the rules an R.C.C. does not get to select an O.C.C. Take the Dragon Hatchling R.C.C. for example. That would be a pretty powerful character there and then there would be questions about which bonuses to add and so forth. So by the book there are no rules and you didn't miss anything.
I ran a powerhouse of a game and let a player do just that. Only we went with the higher of the bonuses and made some other modifications. It was a Shifter Royal Frilled Dragon. He used all the magic bonuses from the Shifter, the base P.P.E. and skills and powers of the Shifter. Other than the dragons natural abilities I did not use any of the R.C.C.'s skills/magic/bonuses. I'd have to talk with the player if you want more specifics.
Hope this helps
Carl
