Angel1, I haven't grown Cornish crosses as meaties (I use some of my dual purpose birds for meat) and I have not done research on them so I can't answer some of your questions. That won't stop me from offering an opinion though. I can answer some questions about raising chickens in general.
If they are eating anything other than chick starter, like hay, grass, bugs, or grain, they need grit to properly process it. If yours are outdoors in sandy or rocky soil, they are probably finding their own grit. For my chicks, I put sand in a separate feeder and let them eat however much they want. I just wired a small yogurt container to the side of the brooder and kept that filled but I did not have as many as you. I found after I started offering them sand, they quit having pasty butt, but I'm not sure the two were related.
You need to know what "meds" are in the medicated feed to know what you are feeding them. Are you trying to treat the runs, the sniffles, constipation, or an infection? Different meds do different things. I'm being blunt there to make a point. I agree with you as you said to only treat them for something specific, not throw medicines at them for no reason. Most medicated feed contains Amprolium, Amprol, different versions of the same thing. Amprolium is not an antibiotic and will not cure anything. It is a substance that reduces the breeding of the protozoa that causes coccidiosis, often called cocci, in the intestines of chickens. If you don't know about cocci, you can do a search on here and learn a lot. Cocci can kill chickens if they get huge amounts of the protazoa in their intestines. The protazoa thrive in wet manure. So if you keep your brooder or coop dry you probably won't have problems. There are different strains of the protazoa and some are worse than others, so some people do have problems even with dry brooders and coops and even if they feed the feed that has amprolium in it. Chickens can get immune to the cocci protazoa by having small amounts in their intestines for a couple of weeks. They actually need to eat some of their one or two day old poop to gain this immunity and share it with the other chicks. The can still get this immunity while eating feed with Amprolium in it. Amprolium does not stop the reproduction, just greatly reduces it. Probably not as important to meaties since they don't live that long, but certainly important to your laying flock.
As I said, I am not a Cornish cross expert, but I'll offer an opinion. If you have true Cornish crosses, most will probably not live long beyond 8 weeks unless you greatly restrict what they eat. I think that is where the confusion over the 12 hours of feed a day comes in. If they are allowed full access to food all day, they will grow tremendously fast but will eat themselves to death at an early age. That's why you pretty much need to process them at about 8 weeks. Otherwise they will eat themselves to death, getting heart attacks, their legs will break down from the weight, internal organ failure, several different potential problems. If you do restrict how much they eat so they don't grow as fast, they may be able to live a couple of years. It is not a healthy situation though. They are specifically bred to efficiently convert feed to weight gain and gain weight so fast they will have bald spots since their feathers can't come in fast enough to keep them covered.
I would not try to keep Cornish crosses for their offspring. If you take a white rock hen and a Cornish rooster, you will get a Cornish cross. My understanding is that special strains of Cornish roosters and white rock hens are used for the commercial Cornish crosses, so the ones you breed yourself will not grow as fast as the commercial Cornish crosses. The Cornish crosses you get from a hatchery are probably not from the same strains as the comercial birds either, so yours might do as well as hatchery Cornish crosses, but they might not. It depends on the strains of the parent stock.
You could probably get the same results by crossing the Cornish rooster with other chickens, especially other rocks. I'm not sure what the Cornish crosses look like from a coloring viewpoint. They could be using the white rocks so the carcass dresses out prettier. If you pluck a dark colored bird, the dark pinfeathers are very unattractive but if you pluck a light colored bird, thye are not noticeable. That's why I skin my Australorps and pluck my Buff Orpingtons and Delawares. Or maybe they get a sexlinked chick so they can elect to grow out only the males, which are more efficient than the females as meaties. Maybe a combination of these reasons. As I said, I am not a meaties expert.
Hope this helps some. Good luck!
