One more question, I've been looking at gear reduction for the stepper motors / belt system.

Have you ever come across an 80T + 20T combined idler pulley? Or a 60T + 20T combined idler pulley?

Running a 20T GT2 pulley from the stepper to a 60T idler that is connected to a 20T gear turning the drive belt would give me a gear reduction of 3:1, which if I run the steppers at a good voltage could give me better native resolution and better / smoother performance.

Alternatively I've been looking at 16T gears, but the idlers only have a shaft of 3mm, and that seems too small for my liking.

It looks like most people are simply running 20T gears without any reduction for their pulley systems?