I just recently changed the chain on my 2001 bianchi eros. The drivetrain is Campy mirage 9 speed, the old chain was a "Z" (I believe), and the new chain is a SRAM 9 speed. Since I put the new chain on, I noticed the paint was scraped down to the metal on the driveside seatstay. In the meantime, I have stopped using my smallest cog (the only position where it could possibly rub). Any suggestions on how to remedy this problem?

Maybe the wheel isn't fully seated in the dropouts. That happened to me once, but I caught it before I rode anywhere. It felt nice & snug, but the axle just hadn't gone all the way.

It sounds like you might need your wheel spaced a little more on the drive side to move the cassette a little further inboard. Or it might be installed wonky, as Barracks suggested. Check it out - it would be pretty easy to pop another spacer on the drive side if need be, and you might not even have to redish.

Heck, I'll take chain rub over knob rub anyday.