Don't worry so much about hardiness. I am in Washington, on the Olympic Peninsula. We get lots of clear days in the winter where the temperatures drop into the low 20's (or colder) at night and do not come above freezing in the day time. The cold spells like this can last as long as a couple of weeks. I have yet to have had any problems, other than having the normal ones of hauling water. I had two big screened windows on my coop, that had no glass so they were open all the time. We also get some pretty strong winds that can last for days, be it freezing or not. I had no frozen combs either.
The big thing I did notice was that they laid less eggs when it was freezing. They seem to be fairly tough. One hen hatched one chick late this summer. She started ignoring it when it was about 4 weeks old. It was in with my flock of 7 hens and a rooster. That little girl grew up just fine. Not that I would recommend doing it this way, but, I could not find any chicks close to her age to put her in my brooder with when the hen quit taking care of her. She sure held her own among the flock.
I need to find some chicks or eggs to add some out crosses. I guess I should start searching more diligently now.