You can. In terms of cell count you'll want to use about 1/5th of the cake. Although I don't know of any yeasts that work well for a scotch ale and a cider, but that doesn't mean there isn't one.
I wouldn't worry about washing unless it was very hop forward. One pint of beer in 5 gallons of cider isn't much.
If you do decide to water wash your yeast it would be better to just add water, chill and decant a couple of times. Save the thick slurry after it all settles, not the part that doesn't settle as described in most "yeast washing" explanations.