The type of short plug your looking for, is called a "dry seal".
If you want the plugs to fit properly and always be the right depth,you need a thread gauge. It's a hardened gauge, that has a flat on it, that involves 3 threads. The thread is the right depth, if it threads in to the middle of those thread but, will be in tolerance, if it align with any of the 3 threads.
So the tolerance for all pipe threads (at least under 1"), is nominal, +/- one thread.
I hope that helps.