Your 11 cruises credits will put you in the Elite status of the Captain's Club with Celebrity. You will be entitled to the Elite benefits. I believe you will need to call Celebrity and let them know about your RCI credits.
What you can't do is combine credits to put you in a higher category. For instance if you had 6 with RCI and 5 with Celebrity you can't combine them to move up into the level that is 11 or higher.
Royal Caribbean Cruise Lines doesn't own Celebrity. It's the parent company Royal Caribbean Cruises Ltd. They are RCCL. The cruise line is RCI. It's confusing. Just as Carnival Cruise Lines doesn't own Princess and the others , its Carnival Corporation and PLC. that owns Carnival, Princess, HAL, Costa, Seabourn, and Cunard.