I rode the GAP on regular 25's, no real problems, but some areas were a little squirrelly either because of mud or un-tamped crushed limestone that must of been just laid or something. People tell me (and I believe them) that the C&O is a little more rustic.
I'm sure your tires will be fine, if I had to do it again, you could get me to trade up to 28s but no bigger. some folks just love big honking tires, I'm just not one of them