Good idea on the heat mat. I used one as well and they liked to lay on it. When I got my chicks we were still seeing temps in the 30s at night so I was scrambling to keep the chicks warm.
My chicks were 3 days old when I got them from the hatchery. All I used was heat plates. One store bought and one homemade from a heating pad. The 3 day old chicks would go under to sleep or when they were cold. If they got too warm, they would come out. They did fine, all eight started...