Ok.... shorter or longer striker....
You would knock the timing out that the marker was designed for, for instance: -
Using a longer strker - the valve would be open before the bolt reached its true fully forward position and the cycle would not work correctly. The paintball would be half fed when the air was released to fire it, so the left over air would just about push the ball into the barrel.
A shorter striker would probably never touch the valve and therefore the striker spring would push the bolt and striker forward but would not open the valve to fire and return the bolt and striker to cocked position.
If the shorter striker did manage to touch the valve, it would be very under powered and you would have trouble with re-cocking the marker at all.
Hope that helped.