You would want to stick with the heat pad that is made for 10-20 gallon tanks, I believe it may be called a small or mini, but it will say in the description.
What you do with the thermostat is this, you plug the heat pad into the thermostat. The thermostat will have a probe that you will place on top of the heat source, which would be inside you tank over top of the heat pad. The probe will read the temperature of the heat pad and regulate it to the temperature you set it at. So if you set the thermostat at 90 degrees, it will automatically cycle the heat pad on and off for you to keep it at 90 degrees. So if you're running the A/C in the summer and the ambient air is cooler it may never turn the heat pad off, whereas in the winter if you have your heat running on high and the heat pad reaches 91 degrees, the thermostat will shut the heat pad off until it goes back down to 90 degrees.
The paper towels should not burn, especially if you are using the thermostat to regulate the heat. I keep all my hatchlings on paper towels in rubbermaid plastic tubs directly on heat tape and have never melted the plastic or burned the paper towels.
If you keep the humid hide right next to your heat pad it should do fine humidity-wise, you will see condensation along the inside of the hide which is what you want. As long as you keep the moss damp (not soaking wet) it will be fine. What I like to do with my moss is I put it in the hide, fill it with warm water and then squish out all the excess water until it's nice and damp.
For the hides, you want to make sure they are large enough to fit a full size gecko, so look at the dimensions of the hides you are looking at buying and go from there. Say you end up with a male (as an example because they are generally longer), you want to make sure an 10" gecko (going with the extreme length) will fit comfortably inside. They will generally curl around the shape of the hide, so you don't necessarily need a 10" long hide, but a good 6-7" minimum is not a bad idea. You also want to be sure that they will be able to fit through the opening when they are full grown, so a 1" diameter opening is a minimum for a full grown adult.