The easiest would be to use chassis inserts suitable for your pinion. We don’t use inserts to adjust the mesh of gearing.
So when placing the pinion on the shaft, there still has to be some backlash(clearance) between pinion and spur gears. So not absolutely tight contact.
The gears has to be in touch with each other with as much teeth surface as possible. The pinion should be spinning smoothly without cogging or too loud sound.
After the gearing is meshed properly, you should also glue the pinion to shaft using fixation glue like blue Loctite.