It's true that a Tiagra might not work with a 32T cassette. 32T is the 50/50 chance for many derailleurs from Shimano that state 27T as the maximum cog. Same holds true with the Ultegra RD. It might work - it might not. If it won't handle a 32T Tiagra, it's time to look for a true mountain-bike RD, such as an XT:

http://www.amazon.com/Shimano-M772-S.../dp/B000YBCNAA

I'd try the Tiagra first, but it's right on the edge where various parameters of the frame, chain-line, etc. - come into play making it iffy at best.