Thermal pads

I'd stick with the same thickness, because if you'll add more, the heatsink will be not fully contacting with other components as it should. I used Phobya XT Thermopads - really efficient, comes with different sizes and thickness.

There seem's to be 2 thicknesses overal ranging from 1 to 1.5mm

Thermal Grizzly Minus Pads are a pretty good option in my opinion, and if you want to be safe, buy different pad thicknesses and then try out what works best while performing some benchmarks!