You will never find a White Cornish sire for sale from any hatchery anywhere, as they do not want us making our own meat birds. But you can certainly make your own meat birds using Dark Cornish sires, which is used ubiquitously in England for backyard cross breeding.
To answer your questions:
You can certainly become largely self sufficient in the chicken category. At first, breeding yourself may be daunting, so I would bring in day old chicks until you get the hang of raising, finishing and processing your birds.
I would start with getting Cornish Cross meat birds first and learning the process of raising meatbirds (and all the associated pitfalls with overly specialized meat chickens). You will probably not find dual purpose breeds of chickens very satisfying as meat birds, so I would recommend you get specifically bred broilers first.
As for raising, if you have limited space, you can raise meat and laying chicks together, as the feed up to 8 weeks is identical. Ideally, though, you manage them separately. Most backyard people like us raise meat chickens in a "chicken tractor" (google it).
For meat chickens, they are usually dispatched between 6 and 10 weeks. 8 weeks is considered ideal. At that point, you will hardly be able to tell the pullets apart from the cockrels, as they aer still very young (but may be pushing 10lbs live weight by then).
"Tender" is a loaded term. You can create tender chicken by confining them in cages and never letting them move. Our ranged chickens will always have a different meat texture, which is stringier, stronger flavored and perhaps 'tough'. But once you get used to it, you'll find grocery store chickens to be insipid and 'wet'. This is because to make chicken 'tender and moist' commercial chickens are 'marinated' in salt water before packaging, which is why everyone says "wow that chicken was tender!!". But they're really being duped and are buying water at the price per pound of chicken.
Anyway, read many of the posts in here, you'll get some questions answered.
