I butcher my extra roos from whatever I have, and that's generally either heritage breeds or mutts. Some of the mixes get pretty meaty by the time they're 20-25 weeks old, some never get very big. They all eat a combo of layer mash, corn, distiller's grain (to boost protein) and black oil sunflower seeds, a handful of dry cat food or dog food here and there, garden scraps, table scraps, whatever they can forage, as they free range daily.
They very seldom have any fat on them to speak of. They just don't get fat like broilers do. I've even put them on extra carbs in a separate pen for a couple of weeks (to spare the hens, the roos get to be a real nuisance, but figured I may as well fatten them up) before butchering, and they were still just as lean.
They taste great though. My DH never cared for stewed chicken, but he loves these crock pot-cooked roos. The flavor is wonderful, and the broth is amazing.
You might do a forum search on cooking these kinds of birds, to get ideas well ahead of time, so you won't be at a loss about what to do once they've been sent to freezer camp.