The replacement is usually 1mm for VRAM and VRM (if you choose to go with thermal pads). You can (as an alternative) use thermal paste for everything as well (MX-4 seems to be the commoner for that) without issue.
If there was thermal pads, you need to use thermal pads, thermal compound will not provide proper heat conduction for the ram/vrm. If you are not sure on the thickness it would be better to go on the thicker side, provided that the pads are all the same thickness. otherwise some parts may not contact the heatsink properly.