I experimented with them all (21, 15ES, Duetto, and Mini). The 21 cuts the fastest of course, but it doesn't matter which one you use. I only used Zephir on the blue pads and Keramik on the yellow...
I believe the first part. Scholl Concept's SE3 Gold works better on Rupes blue pads than Rupes Zephir. The second part? Not sure how to advise on that.