speaking of tube interchangeability, i've found that in a pinch (no pun intended) that i can use a 700c tube in a 26" or 650c tire.
i had no choice when i found myself with a flat on my mtb and a faulty valve stem. a few minutes later a good Samaritan (god bless him) offered me a 700c tube, at first i was reluctant to accept the offer, but i thought, "what the heck. i'll give it a try". installing it required me to fold over the tube and inch or two and i thought for sure i'd notice a terrible thumping during the remainder of my ride.
when it wasn't forthcoming I was more than surprised. now i just carry a spare 700c tube on all my rides.