20 above 0 is nothing to a chicken as long as it's dry and the coop is well-ventilated.
20 below for an extended time is when you start thinking about heat. :)
Sounds like you've got some great chicken personalities in your flock.
Fully-feathered, adult chickens shouldn't need heat in their coop unless you live in a truly extreme environment. You can add your general location to your profile so people can give targeted advice. :)