Depends what you mean by "compatible" ....
6 and 7 are compatible in the sense that they, in the case of Campag "Screw-On" type systems, where the freewheel screws onto the rear wheel, can use the same rear dearilleur.
In early versions these used a friction lever and the only things that sometimes needed a change to the chain, the rear axle spacing (and occasionally therefore some cold-setting of the frame aws needed) and of course the limit screws on the RD.
The downtube-lever indexed versions of 6 and 7 speed (using the infamous "Synchro" and "Synchro 2" systems) need different inserts in the lever but could use the same RD.
9 and 10s are interchangeable in the sense that the cassettes will fit the same rear wheel (to correct AndyRM, NO spacer is required in the case of Campagnolo, the cassette is designed to fit straight on - unlike Shimano where a spacer is needed under the 10s cassette) and the same RDs can be used for 9 & 10s provided the RD was made after model year 1999/2000 - 10s compatible RDs can be identified in that they don't have a "B-Tension" screw at the mounting point - however, you can't use a 9s ErgoPower lever with a 10s cassette or vice-versa, without modifying the internals of the lever - in many cases this can be done but not in all.
HTH
Graeme
Velotech Cycling Ltd, Main Campagnolo Technical and Service Centre, UK
