First off, Mayweather will never fight Pacquiao unless Manny get knocked out or severely damaged by another opponent. Mayweather is a purebred coward is sole purpose is self-preservation and compensation, not being a warrior and taking on the best. Maybe if you're old and lost a step or two, or been beaten up in previous bouts, maybe then, you could get a Mayweather fight.
Secondly, Mayweather's style is not one that is ideal for Manny's. Pac's work rate and energy level are just too high, and Pac will throw more punches, as well as land more meaningful ones. Against Clottey, Pac threw 1231 punches and landed 282 of 682 power shots. All this against a guy that he knew could knock him out. Pac and everyone else knows that Mayweather is not a power guy, and will easily out work him if it goes 12, IMO.