In need of a small, heated waterer for small coop (6 chickens)

We currently have the main coop which has a big heated waterer and big feeder. We just got a second coop/run for our 6 "naughty" hens--great egg layers but not getting along with the younger pullets (this coop will be our breeding pen eventually).

The coop isn't large--5'X4' and there isn't really room for a gallon waterer inside. I put one of those in the run for them but only have my baby chick waterer in the coop (which isn't really adequately sized for their beaks).

Would think work in their coop as a heated waterer on a cinder block for the winter? Can chickens drink out of this? I'm in the coop twice a day, so refilling is not an issue.

Any other ideas for something smaller than a gallon waterer would be great. Thanks!
Sarah

http://www.amazon.com/Innovators-Heated-Water-Bottle-Clear/dp/B001CCZ82A/ref=cm_cr_pr_pb_t