Got lost in all the questions ill answer a few i know
Me i would heat the room to ur temp ur gonna keep it at when snakes are in it then heat up ur racks. I would let it go for maybe 4 hours to adjust and then put them in there. Thats just me.
I would say no hides if their in a rack
Substate i use is this.
http://www.uline.com/BL_1961/Indented-Kraft-Paper
I LOVE it. Its absolutely amazing. You could get the non indented from walmart. But i like the texture of it and its only $35 for around 350 feet of it
My oil filled radiator heater sits at 78 deg. Because it allows my room to still be a comfortable for me and theres always a 2-4 deg diff in my tubs compared to my ambient air temp
As far as humidifier. You can go get a cheap one at walmart? I choose not to use one in my room because if ur using it to much i feel ur prone to grow mold due to the room being at such higher humidity. Maybe im crazy