Hi~ I have currently the following setup:
Campag 9 Speed Record rear mech with short cage (RUNNING 10 SPEED btw)
11-25 Cassette
50/34 chainrings

I believe I have too long of chain wrap for my rear mech (30) and need a medium cage for better shifting and possibly less cross chaining. So, here are my many questions....

1. Generally how much longer of a chain is needed for going from short to medium cages?
2. What parts besides the two halves of the cage do I need for the rear mech? I assume I can go Chorus instead of Record for $$ savings?
3. Is anyone here running this setup, meaning medium cage, 10 speed 11-25 50/34?

Thank you for your help!

-solarpos

Chain length difference would seem to be the difference in cage lengths of the respective deraillers
measured along the chain path. Between 1 and 2" most likely for short to medium length cages.
I just took down what you are working on but had a short cage centaur RD (50-34 and 12-25 in
a 10spd set up), worked fine as it was. Perfect is the enemy of the good. Cross chaining is not
going to be changed by altering RD cage length. Cross chaining is a problem for the chain going from
the top of the cassette to the top of the CW, not the return chain. Centaur is even cheaper than Chorus.
Any of them are acceptable, depending on your bling tolerance. If an Italian frame with nice paint, Record is
the only way to go, Chorus is for wannabes. If Motobecane or Scattante......Centaur.

1. Will vary a bit by derailleur manufacturer, each will tell you how to properly size the chain for their product.

2. Not sure I understand the question, assume you'll just be swapping the entire derailleur?

3. I ran 50/34, 11/25 (10sp) for a long time with a medium cage derailleur. I recently changed out the rear cassette for a 12/27 to make it a little easier on my legs climbing the hills we have around here. Both work fine.

BTW - I don't find cross-chaining to be a big deal running a compact setup.

Guys on the Road Forum say you can use a short cage and just put the chain on the large CR and largest cog and add one link for proper chain length. Sounds easier than buying a new cage or rear D

You definitely can run a compact with a short cage rear up to 26 teeth. I am running a Record 10sp drivetrain, short cage rear, '08 front (so not the special compact cage FD- the '08 and later will work with both standard and compact), 50/34 and 13-26. No problems at all, although I did need to take out a couple of links when I converted from the 53/39 to the compact crankset. Your problem may be chain length; I am not familiar enough with the 9sp RD to know if that is part of the issue.