I plan on keeping 6 if they are hens and rehoming the rest so yes essentially I am asking for sexing purposes but I also want to know how long I have to keep the 17 to get my 6
I've got buff, barred, and partridge cochin
I don't know on the large fowl for that, honestly. I can mostly tell on my bantams at about 3 weeks of age and am sure by 4 or 5 weeks of age, but large fowl tend to be slower to mature, so you might have to wait 6 weeks or more for the slower to mature males to pop up.
You should be able to tell by feather coloring in the Partridge a little earlier; females are laced and males have a black chest with reddish patches on the shoulders. Barred answered below, but Buff you'll probably have to wait until 6 weeks.
Can I sex the barred cochin like you do barred rocks based on coloring once fully feathered?
Yes, you should actually be able to tell by the feather coloring like with Barred Rocks. Males are lighter than females in a pure barred variety regardless of breed. The barring gene is a sexlinked gene, so males pure for the gene have two copies of it, but females can only have one at most. The gene acts as a partial dominant, making thinner white bars with one copy of the gene and thicker white bars with two copies. So, since the males have two copies in a pure barred variety, they are lighter than the females that can only have one copy of the gene at most.