or can you request a single cabin if they are available, or perhaps at some cost
i'm thinking on working on celebrity as a musician
Unless you're a featured entertainer with full passenger privileges, I tend to think you have to share a cabin. The reason is simply because of the space constraints on a modern cruise ship. If any crew member could request their own cabin, there wouldn't be any cabins left for the passengers.
As Rita says - not unless you're a featured guest entertainer. Are you just in the orchestra, an intermissionist? On RCCL ships (parent of Celebrity) the piano bar guy/girl has their own room by contract and so does the musical director. Every other musician has to share.