nope, not too cold. After they've gone to roost, go check them. Pick them up and stick your fingers under their feathers, you will find they are toasty warm.
Not sure about the stall wall top. When they are roosting, look to see if you can see their toes sticking out. If so, you might want to put up a 2x4 flat , so they are on the 4" part. That way, they are basically sitting on their feet and their toes are covered with feathers. My roosts have always been the flat side, so I haven't had this experience myself of a bird freezing its toes but I was cautioned way back when about that potential problem. So could be old folks tale, could be true.
Its a good idea to try to routinely pick up your birds and check them out. roosting time is good for this. I manage this maybe once a month, more if something is going on. It lets you notice someone who is losing weight, etc. Not at all practical when you have lots of chickens, like minniechickmama, but for those of us with smaller flocks, very doable.