I would say it would be very difficult to do a rat's face in GS, depends how much natural talent you have however, it's not the easiest thing to sculpt with in all fairness.
I would say to get any spare skaven heads and cut them down till they fit within the hoods, either that or just make them as goblins and then have a mix of humans, elves ect within the same unit and have them as slaves from other races as opposed to actual skavenslaves.