I've noticed pasty butt more of an issue with shipped chicks and feed store chicks (which have also been no shipped at some point). I think the stress is the cause. I rarely have issues since hatching my own. That being said, I've also noticed if the temps are off, i also have issues with pasty butt. Typically if it's too hot. Might double check but I thing your issue with pasty butt is from the shipping.
I've got a thermometer in on the floor of my brooder and due to the fact the chicks have no problem laying directly under the lamp, nor are they all scattered away from the lamp, I don't believe heat is an issue. They go out from the lamp to eat and drink, but they come back under to sleep, they don't huddle together in one clump either, you might see 2 - 3 sleeping together, but there will also be several individuals sleeping alone. I don't think temperature is a concern either too high or low. Thanks for the tips and information though.