I have a very large 10 year old corn snake that will not eat rats. I have been feeding him retired breeder mice for two or three years now with no apparent problems.
I don't think retired breeders would be lacking in calcium. My concern was that they would be too high in fat. When I decide to retire a colony I take the male out. I then wait 3 weeks to make sure none of the females are pregnant. After they have raised their last litter (or I have harvested it) I feed the females a low fat diet for a couple of weeks before I feed them off. I don't know if all this is really necessary but I feel like it's a good idea.