This opens up a whole can of worms ...
As AyeYo says, you can't fit a Shimano cassette onto a Campagnolo cassette body - the spline pattern is totally different.
SRAM will fit to Shimano - that's it, in terms of cassette / cassette body compatibility.
Campagnolo 9s systems will index tolerably well on 9s Shimano cassettes but it's not perfect and it needs to carefully set up - it has limitations and we would not recommend it and it should not be recommended by retailers or anyone else who may have to carry liability if it all goes horribly wrong..
Shift levers are not affected by max and min sprocket size.
Rear and front derailleurs are affected by sprocket size.
Campagnolo do make a 10s long cage rear gear that will work with the 9s levers (the geometry is the same if the levers / existing rear gear are post 2001) but it's maximum rear sprocket size is 30T (see Campagnolo tech docs at www.campagnolo.com)
The shop were not misinformed, within the limits of the info they gave to the OP they were correct - in addition, what has to be taken into account from any commercial organisation is this - once advice is given that falls outside of the manufacturer's specification, the liability, should there be a problem, rests with the individual or organisation giving the advice - so unless a specific caveat is quoted (as I have done above), a retailer or other commercial party is potentially laying themselves open to all sorts of problems by adopting a gung-ho "yeah, it'll be fine" attitude.
Commercial's shouldn't assume knowledge on the part of the customer in these situations ...
To give an admittedly slightly unrealistic example -
Customer Q: Can I fit wheel "x" with carbon rims into my road bike?
Retailer answer A: If they are 700c and the cassette is compatible, yes.
Time passes
Customer (returning some hours / days / week later): You said I could put these carbon wheels into my bike - but when I hit the brakes in the wet the other day, I couldn't stop and ran into a brick wall and scrapped my frame forks and wheels - I could have been killed ...!
Retailer: Did you have carbon-compatible brake blocks made for those wheels fitted?
Customer: No, you told me I could fit the wheels - you never said anything about the brake blocks!
Retailer: Hmmmmm
OK, so any retailer would have spent time making sure that they had all of the info (unless they were mad) ... but you can see how slippery the slope can be - we do see answers like that on fora all the time from respondents to posts like the one from the OP.
HTH
Graeme.
Head Tech & MD, Velotech Cycling Ltd
Industry Recognised Training for Cycle Mechanics | Campagnolo main UK Service Centre
Technical Director, Service Corps Ltd
Cycle event servicing and logistics