What’s the compatibility of a 10-speed rear derailleur with a 9-speed drivetrain? If cage or pulley width is an issue, what about swapping the 10-speed pulleys with 9-speed pulleys? Do they fit? Would I need the longer bolts from a 9-speed derailleur? Will any of it work?
If you use a Shimano or SRAM 9-speed chain (or a 10-speed chain), it should be no problem (assuming all of the parts are Campy). You won’t need longer bolts, since all of these chains work with Campy 10-speed.