Using a twin XL comforter of 68 inches wide by 90 inches long on a full size mattress (54 inches wide by 75 inches long) will give you approximately a seven inch overhang along each side and a little less at the foot of the mattress once the comforter is pulled up to cover the pillows.
If you plan to use a regular twin comforter, the size can range from 66-68 inches wide and 86-88 inches long. This would not allow for much overhang when placed on a full size mattress (54 inches wide by 75 inches long).
Have used a twin flat sheet for the top cover on a full size mattress, particularly during warm weather months when need for coverage is limited and overhang not required. Pillows are the same for both mattress sizes, so standard pillowcase will work on either. Because the sale prices can be good, often will used the longer queen or king size pillowcases, placing the pillow inside and tucking the opening excess material to form an envelope finish.
Yes they can
They can but it wont fit very well
