I have two buff hens for eggs. I didnt think about cooking those. So pretty much their boilers are cornish just colored well dang. If they free range with our hens would they grow slower and be healthy still? I give ours a scoop of feed mixed with scratch(its a large scoop used for horse), scraps, the rest they find their own food they barely finish what i give them between 21 hens by the end of the day.
The Buffs you have are likely Buff Orpingtons. True Orpingtons make great meat birds, but hatchery Orpingtons are very watered down and much smaller and skinnier than their SOP counterparts. I have eaten hatchery stock Buff Orps in the past, and was not impressed.
Free ranging Cornish Cross is not generally the best idea. They cannot even run, and will be easily taken by predators. They may grow slightly slower but it's not likely to be a noticeable difference. They may or may not still experience health issues. Personally I believe the health issues experienced with Cornish Cross are often the fault of the keeper, not the birds. I've been raising them for several years and have yet to have one die for any reason besides butchering or predator attacks. Not one heart attack, keep over, or mysterious deaths. My CX have access to 4,000 square feet of run, 256 square feet of coop, and broiler-diet available to them for around 8-10 hours a day. I've had batches survive 90F. heat waves without breaking a sweat.
Remember that you will want to switch the flock to a broiler diet when you are raising the meat birds. And you will probably want to set ip a second feeder so the broilers/meat birds don't jostle the hens or vice versa. If you choose broilers, you will want to use a feed around 20% protein. If you can find a non-medicated starter feed or a grower or broiler feed with similar protein content, that will be perfect. If you choose heritage meat birds, e.g. Standard Cornish, you will want to feed something with around 18% protein. Either way you will probably be feeding a starter, grower, or broiler feed, which will lack the calcium required by laying hens; this need is easily supplemented by throwing out a handful of oyster shell every few days. Alternately you can purchase a sma (1-3 pound) feeder and allow them free choice oyster shell from it; or, you can simply mix it into the feed if you wish.
Also just to be sure you are not misunderstanding anything I am saying. The diffeence between Cornish Cross and Standard Cornish can sometimes be difficult to grasp for those new to meat birds. A Cornish Cross (aka Cornish Rock, aka CX, aka Broiler) is the industry-standard fast-growing broiler, the product of 60 years of innovation. They grow to butcher weight in 6-8 weeks and have the potential for many health issues. The Standard Cornish (also simply known as the Cornish) is the bird used to create the Cornish Cross all those decades ago; it has a similar body type but grows much slower, taking 20-24 weeks to reach market size, and does not have near so much potential for health issues as Cornish Cross do.