Gear reduction really depends on how fast you want to go, how fast your steppers are prepared to go, torque required for accelerating/decelerating the axis and rolling resistance.
As your steps per second increase your torque falls away, there has to be a limit to how fast you can reverse the current in the motor coils and still get useable torque.
Unless you have a graph showing revs/second against torque, for a small motor I'd suggest holding the motor shaft between finger and thumb as a rudimentary friction brake then wind the speed up until there is no useable torque left. That gives you a number to work from.
Calculate a gear ratio that gives you the required inches per second then work back to find your step resolution and be prepared to compromise.
My 3.6" Escap steppers, wired in series with a 24 volt drive, clack out around 5 rps in half step, useable torque is fading fast by 4rps. I want 200 inches per minute, 3.3 inches per second so 1"/rev looks about right. Half step resolution 0.0025".
For this kind of power most any pulley belt combination is fine. The real load comes when you stop the motor and it applies it's full holding torque while the axis wants to keep moving. Unlikely to shear teeth off the belt, but it will try to ride up and apply a severe side load on the shaft. I know small pulleys are cheap, but can you get enough shaft inside them so it isn't going to bend?
