Change frequencies Read this thread here first. Are all steppers microsteppable?
What kind of drivers do you plan on using. In your example above, you talk about .000125 @ 1/8 step, which is about 8000 steps per inch. Mach2 can deliver up to about 45000 steps/second. thats over 300 inches per minute.
I would gear your motors down 2:1, or get different leadscrews with 10tpi, and use 1/8 steps. You'll get smoother movements, and still get the resolution you need. You'll also double your available torque.