It does not matter where you get the Cornish cross, they all came from the same breading stock, Most if not all hatcheries get the eggs from just a hand full of breeders. There is at least two other Cornish cross line I have seen but they usually take at least 10 weeks and are a bit smaller.
Yes there is a couple of different types of Cornish cross out there. But the ones they say that can be ready in 6 weeks are the same. We gotten Cornish cross from MM and Estes and they are the same.
Last year we did not seperate them and the other birds would sit on the Cornish cross. This year we only had Cornish cross as meat birds and are in the process of butchering them now.
If you don't use the 12/12 feed method, then about 6 weeks is when they should be butchered. If you do use 12/12 feed method, then any where from 6 weeks to 10 weeks. Ours are at about 8 weeks now and I noticed a lot a difference in there internal organ size. We only did the 12 /12 method for about 1/3 the time we had them.
We put there water and feed on oppisite ends of the pen which is 8 feet wide and about 20 feet long for 26 of them. That made them get up and move around some. Althought there leg meat was a bit darker we only lost two this year. Those were lost when we didn't do the 12/12 feeding.
Tom