one of my bikes uses Campy Centaur brifters and front mech with Shimano XTR rear mech (and a 9-speed 105 cassette).
so mixing can be done.
the only thing i'd make sure of when choosing a rear derailer is low-normal vs. high-normal actuation. both will work, but a low-normal (Rapid-Rise, in Shimano-ese) will shift the opposite of a standard road derailer.
afaik, all Shimano rear mountain derailers are available in both low- and high-normal.