If they are feathered out, they should do okay. As you said, the residual heat will last quite a while after you shut them up, and if the low is 60, it'll probably stay warmer than that in the coop anyway, even during the wee hours. It is a little cooler than the recommended 5 degree drop per week, but I think they'll do okay; they can snuggle together in the bedding. Now if you guys typically drop below the forcasted temps, then I might not do it yet (we live out a ways, and we're almost always @ 5 degrees cooler than what's predicted for nightly lows).
