Hi blackrider:

The output from the trackball is a rectangular signal and you don't normally need a Schmidt trigger. If you needed one, there is one inside of many "PIC's".
Is this the only thing you want to do with the PIC? The 18F2620 would be too "BIG" for this project. Also, what is the approx. frequency coming from the mouse and what is the time frame for the signal to be measured, like pulse per seconds, minutes etc?