Having just read a bit more on the topic, the drive roller would be fine in 7075T6 with the idler in mild steel - had stainless in mind as it's what (little) I remember from the college art room... The crucial bit is that the rollers run absolutely parallel to each other (this can be tweaked via the take-up bearings) and that the running gear can withstand high pressures.

I may well have to consider increasing the shaft diameter to a more substantial 20mm and have a closer look at using self-aligning bearings to accommodate said tweakage... This could get a bit pricey...

Wal.