As long as both spur and pinion are of the same PITCH (tooth size), any pinion will "work" on any spur.  Whether that gives you the best gear ratio or not then depends on the size of your wheel-tire combo, the motor you're running, and the terrain you're running on.  Usual recommendation is to start with stock gearing, then go up or down a couple teeth on the pinion.  Changing pinions means you'll need to reset the mesh of the pinion to the spur by loosening the motor mount screws and moving the motor so the pinion is closer or farther from the spur.  Setting mesh too loose can result in stripped spur teeth. Too tight, and the motor bearing will be stressed.