On fitting 700C rims to a dynamo hub rated for up to 20"
Generator hubs for small wheels will output less and produce less drag at a given speed when built to a 700, because they'll be operating at fewer RPMs than expected.

AFAIK all the non-sondelux Schmidts still have output suitable for halogen, including XS-M. You should double check when you order the hub. If so it will pair fine with LED in a larger wheel.