I really don't remember for certain whether it was the long or short screws that came with mine, but the specs call for the screw to not protrude through the mount more than .095's. That's about 3.5 threads I think. As for the washers, I don't recall using any of them.
That .095 length of the screw is critical so as to not interfere with the slide working. But be sure to clean the screw threads and the threaded holes in the slide with a little alcohol, then add a dab of blue "Loctite" to them before tightening to IIRC 15 in lbs.
No quick, easy access to my Hellcat right now to look, but that's the way I remember it.
While waiting for my Shield RMSc to arrive, I purchased the screw set from SA ($4.99 shipped free) just to be sure. Install was a breeze and after about 1000 rounds I've had no problems. I did use a tiny drop of blue Loctite after an alcohol cleaning and torqued to the recommended 15 inch pounds.
My Dad's RDP came with the SMSc mounted by the dealer but they didn't use the small nylon washers that appear to be for the SMSc mounting screws (they were in a small bag with the sight adjustment dial).
Are those washers required to shorten the screw length as to not interfere with the slide internals?
He hasn't fired the gun yet and it function checks properly.