Skimming raw milk is a pain. If you use a separator, it takes heaps of cream to prime it and is a laborious cleanup.
Letting the cream float to the top and ladling it off can reduce fat percentage significantly, but my experiences were spotty.
Mixing ladled off raw milk and commercial skim is a possibility.
I found i was getting a really good result making full cream Romano and Manchego and long ageing, as well as a much higher yield.
If i was making Parmesan again, I would seriously consider buying commercial milk.
I never pasteurise my raw milk, but I am absolutely confident about the source (this morning was Winnie and Monique, the cheese today is Cotswold) and if you have even the slightest doubt or misgivings, low temp long hold pasteurise to make sure.
Raw milk is just the best to work with, and the cheeses are fantastic.