The Ballscrews on the market are mostly metric. The description is usually Diameter and Pitch. Those types is common - 1605,2005,2505.
How to read those numbers: For example if number is 2005, then diameter of spindle is 20mm, and pitch is 5mm.
For Steppers it's best to go for full torque and best speed -> that means 400Steps / Revolution or half stepping.
This will end up in a 4 on 5 System that means 1,25 DIV / Mult.
The difference between Gearing and Timing belt is mainly direction of movement.
Those are good combinations:
1) Tiny steppers with 1/8 (6,25mm) 20/25 20 on stepper 25 on axis spindle.
2) The 1,25mm to the system 32/40 teeth that will fit bigger shafts.
**Precision => you then attach 0.01mm (0.00039") that fits the European precise 1/100 System(metric) that is most used in CNC Stuff
I made a animation to that to show the effect (double press the play 5sec delay to Cyclefinish)