By all means try it at 100 - I reckon it probably does 100 in Xtol, but maybe only just. 50 will be safer for your shadows. Shoot a sheet at each (or a few frames at each, for rollfilm) speed and see which of the negatives you prefer.

Keep the same development time initially. The recommended times give about the right contrast I think but if you're not accurate with your developing time/temp/dilution, it can build to too-high contrast pretty quickly.