I've got a short cage 9 speed Campag rear mech that I'm running a 13-26 cassette on atm. Can I run a 28T cog on it as I'm running a 34 chainring at the front, or is the mech size the limiter?
If it's a 9s generation RD, short cage, the max sprocket size is given in the tech docs as 26T - however, if the hanger is on the long end of the 24-28mm range recommended by Campagnolo (same spec as for Shimano and SRAM) it is sometimes possible to push it to 27T.
The overall capacity of the RD to wrap chain, though, is what really governs what you can and can't do - in the case of these older style short cage RDs, that is 30T, so if you gave, say, 34/50 at the front (difference = 16T), then you only have 14T difference to play with at the back - 12-26 or 13-27 for example.
Changing the cage does not change the maximum sprocket size (that is governed by the design of the top of the jockey cage and that's the same between shot, medium and long cage) but it does change the amount of chain that is wrapped, so a medium cage gives a capacity of 32T (which means 34/50 and 11-26 would work) and a long cage gives a capacity of 36T, making a triple at the front, 30-40-52, possible with 12-26.
Another solution might be a later version 10s RD, depending on the age and lever type of your 9s system. Cable pull ratios changed in Campagnolo RDs when Campagnolo removed the "B-Screw" at the hanger and moved to the "H-Screw" on the jockey cage. The lever type is also important - you will need to be using an "Escape-type", that is, one gear at a time shift to the smaller sprockets, multiple shifts possible to the larger, not because of cable pull ratio but because of the springing in the RD that pulls the RD towards the smallest sprocket - the 2010-onward versions ("11s shape") of the RD have a weaker spring designed for Escape & PowerShift and don't work reliably with "full ErgoPower" levers that allow multiple shifts in both directions. Sometimes you can "get away with it" but it depends on cable runs and absolutely minimising froction every where in the cable path.
If moving to a "new" 10s RD is viable on those points - you'd need to run a 10s chain so that it would fit properly through the jockey cage but that has long been a Campagnolo factory recommendation when keeping 9s systems serviceable - the later (i.e. current) 10s RDs have a 30T maximum sprocket size and wrap more chain, giving the possibility to use 34/50 and a 12-30 cassette. The cable pull ratio is the same as the 9s RDs with no B-Screw, from 2000-2008 (when 9s officially dropped out of the Campagnolo range).
