Obviously, since the circumference is larger on the 26" fenders its not going to fit very well. It will come down to what fenders on what bike.

I have some of the quick removal type fenders on my MTB, with some adaptation they might fit my Dahon but would look silly since their arc does not match the wheels and might not be very effective either.

Maybe you can find some quick type fenders for 20", I have seen them on some BMX bikes.