I just changed out my skimmer (the old one was cracked).
When I put my liner in, I cut the wall foam back from the edge of the skmmer (and return) opening. - as per the wall foam instructions. You do not want wall foam behind your skimmer face plate or your return.
My old skimmer had a two piece gasket, which I used again when I put it back in during my liner replacement. I asssembled the entire skimmer, and then cut out the liner.
With my new skimmer, with the butterfly, or "wall saver" gasket, the included instructions (Hayward) said to first mount the skimmer with the gasket in the inside of the pool, and cut out the liner - being careful not to cut the gasket, then remove the whole skimmer assembly, place the gasket with one half on the outside and the other half on the inside, sandwitching the liner and the wall between the two parts of the gasket and then remount the skimmer. This ensures that you get the holes for the screws in the right spots in the liner.
When I replaced mine, clearly I did not have to cut my liner again. The gasket went in over the edge of the wall and liner, and lined up with the existing holes. There was no extra gasket for the faceplate - all it came with, and all the instructions mentioned was the butterfly (which they call wall saver) gasket.
-dave