Heat lamp temperature

Cold hardy means they deal well with cold temps. All your breeds are,you should never heat assuming your flock is healthy and have protection from wind and wet. I've never heated, even with a not so cold hearty breed, and temps in the negative teens without issue. Ventilation and dry air is much more important.

 

Yes I agree. No heat needed! Check out this great article on ventilation...the most important thing in the winter: https://www.backyardchickens.com/a/chicken-coop-ventilation-go-out-there-and-cut-more-holes-in-your-coop
I know its hard not to worry, but just picture all the wild birds out flying around in cold temps....and chickens are much bigger than them! Keep them dry, with plenty of ventilation and fresh air and high quality food and you are good to go!