CC pretty much covered it. Grain only diet will not be sufficient for a flock of either layers or meaties. All will have growth and behavioral issues if kept on a diet which does not meet all of their protein, mineral and vitamin needs.
For meat birds, you have several choices:
1. CXR grow out to processing wt any time between 8 - 10 weeks of age. They require careful monitoring of intake to be sure they don't succumb to cardiac or orthopedic issues due to their rapid growth. They will respond appropriately to their environment. Many folks say they are filthy birds, that do nothing but lay around in their own feces, in front of the feed trough, and hoover the feed 24/7. If that's the way they are raised, that is exactly what they will do. However, if managed on free range with feed offered later in the day, they will rise to the occasion, and get out and work for their grub. They will grow out slower, but also avoid the lethal problems these hybrids are noted for.
2. Meat birds bred for free range: Freedom Rangers, and Dixie Rainbows aka Pioneers. There are other birds in these groups, I have no doubt. These birds grow fast, produce a generous carcass, and reach good harvest wt at 12 - 14 weeks. They can't subsist entirely on free range, they will need some quality feed to avoid growth and behavior problems. (as does any chicken) They make acceptable layers. Breed true enough to hold them over for second, and third generations, though size will be lost unless you hold over one of the massive cockerels. I held over a Pioneer pullet several years ago, and she was my earliest laying pullet that year, and was a prolific layer of large eggs.
3. DP: Choose a DB breed of good size. You will get good eggs, and good meat, though the feed conversion for eggs or meat is not as good as the feed conversion for production layers, or CXR. Simply harvest your old layers, and your excess cockerels.
4. Mix and match: bring in a group of CXR as time and freezer space allows. Perhaps start some FR or DR with your layer chicks. If you have a broody hen, you could even foster some CXR, FR or DR under her.
