Rear derailleurs aren't "indexed" but they do vary in geometry from maker to maker. Almost all of Shimano's rear derailleurs share the same geometry and some of SRAM share it too so they will work interchangably as long as the shifter and cassette match. This is not true among other makers and Campy derailleurs may not match Shimano derailleur geometry.

Duckliondog found one combination that worked but it's no guarantte yours will. Try it and see but don't be surprised if you need a shiftmate or a Campy rear derailleur. J-Tek is making a nice living because of this incompatibility.