I know if you breed Cornishes with rocks you get fast growing meat birds, but are there any breeds you breed Cornishes with that don't have the same problems as Cornish/Rock Xs. I am not necessarily thinking of meatbirds.
The Cornish Crosses that are sold from hatcheries are not just cornish mixed with white rocks. They are instead a closely guarded "recipe", all of the best stock they have. So if you are just looking at mixing red-laced Cornish and white rocks, you won't get "fast" growing hybrids, in fact at a 22-24% protein feed, they will still only be a shadow of the "Cornish cross" that you could buy from hatcheries. I'm working on a mix of the 2, Dark Cornish and White Rocks, you can see my progress in the below thread: