We crockpot most of our chicken. Definitely a must do for dual purpose or heritage birds, like the ones you have. Ridgerunner has said it all. Cornish Cross (or CX or Cornish X) is THE chicken you will find in a typical grocery store or would be served at restaurant. If you have any Asian grocery stores near you, you can find other kinds of chickens for meat, such as Silkie (one of my favorites).
We even crockpot CX birds, but not as long. Typically, we will crockpot a CX for 4-5 hours and a dual purpose (usually about 5 months old) for 6-8 hours. Google for lots of good recipes.
Recently, I pulled a Chantecler (known for good meat) Partridge cross from the freezer. We had another of these and we crockpotted it. It was the toughest chicken I have ever eaten. Actually, I couldn't eat it...it was literally like trying to chew rubber. So I simmered this one in a stockpot full of water for about 8 hours. It was of course, falling off the bone by then. It was really delicious, not tough at all. Well, not as tender as CX, but certainly very tasty and edible. I pulled all the meat off and made broth from the carcass. By comparison, if I were to cook a CX in a stockpot like this, it would be ready in under 3 hours.
Somewhere out there, I read an
excellent article about how to cook chicken. This was written long ago, before the invention of the CX, so all the chickens were slow growing. There are definite ways of cooking these slow growing birds. There were instructions for birds that are 2 months old, 3 months old, 4 months old and on up to 6 months. If you are raising them for meat, typically you won't butcher past 6 months of age, because they don't get much bigger and the meat just gets tougher. You certainly can eat most any bird older than 6 months, but you will absolutely need to employ a slow cook method like a crockpot or a long stew in a stock pot and even then, the meat will be delicious (more flavor from age) but a bit tougher than CX.
This isn't the exact article I am referring to, but this one covers much of the same material:
http://www.albc-usa.org/documents/cookingwheritagechicken.pdf
EDIT: this is quoted from the article, it corrects and clarifies what I said above:
"The chicken meat most of us take for granted today is quite different from what our grandparents experienced. Today commercial chicken meat production is very different from methods and ideas common before the mid-20th century. Those of us who want to conserve old chicken breeds need to understand the traditional chicken meat classes and their excellent cooking qualities.
There are 4 traditional chicken meat classes: broiler, fryer, roaster and fowl. The traditional broiler age range was from 7 to 12 weeks, and carcass weight from 1 to 2 1/2 lbs. (Squab broilers would be youngest and smallest of these, typically Leghorn cockerels about 3/4 to 1 pound dressed.) The next age and weight group was called the fryer. Traditional fryer age range was from 14 to 20 weeks, and carcass weight from 2 1/2 to 4 lbs. Traditional roaster age range was from 5 to 12 months, and carcass weight from 4 to 8 pounds. Most roasters were butchered between 6 and 9 months. Hens and roosters 12 months and older were called “fowl” or “stewing fowl” signifying that slow moist cooking methods were required."
I don't recall the exact age, but I think you wouldn't want to try any quick cooking methods (like BBQ) with a bird older than 2 months. Otherwise, it will be tough and you'll be disappointed.
