Headset ball bearing size I replaced my chewed up retainer bearing with a rusted but not so chewed up retainer bearing. What happened to the old one was that it was installed backwards and the retainer itself was completely trashed as it was doing the job of the balls. 

The new bearing is a bit rusty and it's balls are smaller. However, all headset play has been eliminated and steering is pretty smooth. 

Should I get brand new bearings anyway? Are there any practical downsides to running rusted bearings that fit but use smaller balls? Any possible damage to the cup?
Buy and fit new ball bearings.  They're under $5NZ $4US for a set.  Its false economy to not replace them when the headset is open.

Headset bearings are probably best in retainers rather than loose bearings.  They don't roll a lot compared to wheel or BB bearings, and there's a lot of small bearings around a relatively large race compared to wheels.