Design considerations for small snub pulley for small low power conveyor

I have this design for a conveyor. I want to use 50mm and 20mm pulleys.

This is my idea for the 50mm pulley.
Conveyor Pulley Concept

This is all well and good for a 50mm dia pulley (I hope. Tell me if it isn't), but I need to make 20mm dia snub pulleys, is it logical to make the same thing but smaller with a very thin shaft and very small bearings?

Or should I just throw a plastic cylinder onto a metal shaft for such small dia?
This conveyor is not exactly heavy load, but the snubs will be responsible to increasing wrap angle of belt to drive pulley.

I'm also unsure on which layout to use for mounting my central drive pulley (shaded), see imgur link. The textbooks all have the drive pulley mounted to the side of both snub pulleys, but I have seen more conveyors in the industries where the drive pulley is mounted in between.

Can we discuss the pros and cons?

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