Making sure they have fresh water and a draft free coop is the biggest concern. If you don’t have a water warmer just be prepared to have fresh water on hand.

They will huddle together and warm up the coop just fine. For extra measure you can rub Vaseline to the combs to prevent frost bite. I’ve never seen the need for the heat lamps inside the coop. They can tolerate the cold extremely well, although some breeds not so much.