Tire size
 

Will a frame built to take 26" wheels accept 650c wheels? This will be to build up a small cyclocross frame that was initially meant to use 26" MTB wheels.

Thanks.

Yeah (might even take 700c wheels) but the brakes aren't going to line up with the rim and you'll have to squeeze the rear dropouts together because road hubs are a bit skinnier than MTB hubs. Just go for the skinniest knobbies you can find in a 26" size.