My suggestion: Flat only. Twin is very universal, BUT if you find a great sale on doubles, go with them.
Strongly suggest NOT super quality sheets, but quantity is better than quality. Reason is that they will get trashed quick anyway, likely stolen in laundry (if they even have laundry), etc.
Sheets and pillows are GREAT things to send.
Want to cause a smile? Send kids sheets (if you find them on sale). Square Pants sheets on a tough Soldier's bed will cause a smile everytime...(I know its goofy, that is the point ;)