you will probably end up having to run a road derailer as well depending how small the cassette is. i've been running a durace cassette (12/25) w/ ultegra derailer for over a year and its been holding up fine.
I've got a dura-ace on my slalom bike and an ultegra on my dh, reliability is not an issue. The only cassettes Ive ever seen break are plastic s-rams and really cheap shimanos with huge low gears. You can run any old derailleur you want, but why wouldnt you want a short cage or road derailleur?