Ah, had not thought of worms, a very good idea. Can you ask the former owner?
If the keel/breast bone is sticking out or feels kinda sharp, they are thin. And worms would certainly make them thin. Correct that the all eat like they are starved, though.
No they're not supposed to be thin. White rocks are one of the breeds that was often used for meat before Cornish X days. Are they full grown? When you say meal, do you mean corn meal?
If both answers are yes, I would go to a feed store and get some grower and/or gamebird feed. They would...