tube interchangeability?

Well, it depends.

If you are using a 26x1-3/8" tube there should be no immediate problem. If you are using a 26x X.XX tube it will work in a pinch but they really aren't intended to be stretched so much prior to inflation- you are basically taking it to it's limits and as such you should not rely on it being a long lasting fix.

The reality is that you CAN use almost any tube in any tire as long as you can get it in and inflate it without a puncture, but longevity is questionable if the tube departs too much from the wheel/tire dimensions Realistically if you use a 700 tube in a 26" wheel the extra rubber has to be folded in there somehow. It is fairly likely it will get caught between the tire and rim somewhere when mounting it and cause a puncture. And even if you don't get a puncture it may cause a slight bump under the tire which will be noticeable when riding. A too small tube (e.g. a 18-23mm in a 28mm tire) will be stretched more than it should be and isn't likely to last long.

Bottom line: If it works it is a good trick, but it's always best to use a tube that fits the tire whenever possible.

usually that doesn;t work too far from the right size

a variety of reasons can kill it

the width has to be right too

it may not overlap

hard to say

you don;t give widths

wle

Never do that.

Do you really think putting a tube that is too small on will not cause some degree of damage?

Won't work. 26" tubes will be way too small in diameter.

*shakes head in disbelief*