Can I vary power by changing pump count?

Yes, that is one of the multi-pump's signature features.

Three pumps might give you 450 to 500 fps for quiet plinking; eight pumps gets the same gun up to its rated 685+ fps for hunting. Use fewer pumps for indoor or close-range targets to reduce noise and over-penetration; use the full count when you need terminal energy. Always pump consistently for any given target distance so your zero stays repeatable.