Bicycle Mechanics - Campy Rear Derailleur Compatiblity Question

I want to set up a 10 speed Campy drive train to handle a 12-25 (coastal riding) and a 13-29 (mountain riding) rear cassette. I want to use a medium cage rear derailleur which will handle both cassettes, but all I have is a 9-speed Centaur medium cage from after 2001 (after Campy changed the cable pull length specification). Will the 9-speed derailleur handle the 10 speed shifting requirements properly? I do have a long cage Chorus 10-speed rear derailleur and I was also wondering if there were any downsides to running a long cage derailleur with a 12-25 cassette? My intention is simply to change out the cassette and the chain when moving from the 12-25 to the 13-29. Thanks for any info!!!

Also one more question, has anyone out there ever successfully run a 10-speed Campy short cage rear derailleur with a 13-29 cassette?

Yes, your centaur will work fine as a 10s derailleur if you use 10s levers. So no issues. Centaur cages are a medium length, and should accommodate the 13-29 without problems. You could also use the Chorus, so it's your call.

The only downside of long cage RDs is that snide lycra wearing riders who hang around the bike watering holes will look down their noses at you. There's also a very slightly increased risk of damage from the reduced ground clearance, but RD ground clearance isn't much of an issue for road bikes.

Yes, I've set up 10s short cage RDs with a 29, but it was on bikes with a smaller chainring difference (52-42 vs. 53-39) which brought it within capacity. You can use it over capacity if you measure the chain long enough to loop the 53-29 ccombination. It'll probably hang slack if you shift to 39-13, but there won't be harm so you can simply avoid using the 39 with the 13 or 14t sprockets, which most people don't use anyway.