What counts towards the 250 hours required for CPL?
It's just what the regulation says:  you must log at least 250 hours of flight time and at least 50 of those hours must be PIC time in airplanes and 10 hours of cross country PIC time in airplanes.  As for the other two hundred hours?  They can be in any other kind of aircraft you like.

It's actually a very common way to build raw hours.  A lot of fixed wing pilots get rotorcraft certificates in a fraction of the total time than if they started training from scratch in helicopters.  It also works well with gliders, hot air balloons, you name it.