I ordered Cornish Broilers from Welp. I liked that the prices already including shipping so I didn't have to figure anything. I ordered late on a Tues night for immediate shipping, they called to verify the next morning and shipped that day. They arrived alive and well in the early AM that Fri. I ordered straight run and while I don't have the exact numbers, I got a pretty even ratio.
This was my first go round with the Cornish. I lost a couple at 3wks to mistakes I made and one past the usual butchering time. I still put more in my freezer than what I had ordered. I did them in a tractor in KS during Nov and Dec. I didn't get the higher extremes on weight even though I grew them longer. Avg was 3-4 lbs processed weight at 8 wks. Had some feeding issues due to the days being so short, they weren't getting a full 12hrs of light to eat and water issues, at least until I got something figured out that held enough and didn't freeze. If you don't mind needing to give them light, freezing waterers and slower growth because of the feed waste to keep them warm, go for it now.
I didn't have to deal with moisture beyond rain and some icy mornings, just cold and windy. Geez it's been a dry winter. Things are supposed to get a little nasty next week last I heard. You may be happier holding off another month or two. Depends on your set-up.
No way would I do them here in the Summer, too much hassle or in the Winter again, JMHO. But I'm lazy and want the best return for the least amount of effort and investment on my part. Early Spring and late Fall though, yep I'd do it and I'll gladly order from Welp again.
For layers, unless you have any specific criteria in mind, get whatever you like or a mix. We have had many different breeds and know others with many more and all have done fine in our climate such as it is. If you're going to free range a lot, go with more browns, reds, mixed colors, etc. The yearling hawks seem to pick off the whites more.