So as far as the Quixel stuff, I cannot really comment. I’ve not had anything come down with a VT/RVT set of logic inside of it, just some basic master-materials like the fuzz and the basic-basic master-material.
As for the Fuzz + RVT question, it shouldn’t matter. Insofar as the material with the Fuzz in it, the RVT is just-seen as another texture, you can do what you want with that information as you will; it’s just scalar-data; you interpret it as BaseColor, Roughness, etc.
If you put the fuzz before the RVT-sample, that would be proper (to my mind); you don’t want to fuzz the stuff coming from the RVT! That stuff should be whatever you painted in, comes out. If you want to modify after blending with the mesh, sure, you can Fuzz it all you want, but to my mind wouldn’t make sense (generally) unless you have a use-case for it.