I've put 1.25's on 26" MTB rims but it definitely depends on the rim and the tire itself. Tire manufacturers seem to vary the bead enough that some tires may fit and some may not. Personally I think the 1.5's give an all around better ride- not quite as speedy but a bit more comfort and a bit more of the right overall tire/wheel circumference for the geometry of the frame. Just make sure they are pumped up to the max. pressure on the sidewall.