Thermal grease is mostly zinc oxide paste, but is not the same as the zinc oxide u get from a pharmacy. The thermal grease has less "grease" in it.
when u place a small amount on top of a cpu and then press on the heatsink, you want to wipe up any excess that flows out.
so in most cases that can be hard to do without making a mess. So its very important not to place too much on in the 1st place.
When placing the thermal grease on, you dont want to see lumps or mounds of it. It's safe for your finger. So take a dab on the tip of ur index finger, about a 1/4 inch circle that's 1 millimeter thick.
Smear it on, starting at the center, and make a little circle outward. Then rub left and right up and down on your horizontal surface, but never go past thee edges. Try to get that dab all evened up.
as for using both pads and grease for the same application, no, they were designed to be used separately.