I'm looking at getting a Soul Cycles Hooligan when they go on closeout. The only problem is that the 2 avaliable sizes are 17" and 19". I ride a 59cm (seat tube and top tube) road bike and find it quite comfortable. I know next to nothing about mountain bike sizes so I'm mostly just guessing.

The geometry listed on their site says TT of 22.7" (57.7cm) and no spec for seat tube. My inseam is 35ish and the site says: 'Standover At Midsection Of Top Tube And Forward Of Seat Tube: 31", 28"'. Do you folks think I'd get away with a 400mm seat post adjusted to match my road bike plus a longer stem, or should I be looking for a larger frame?

This will be doing double duty as a bad weather commuter and trail bike.

For trails, you definately want some clearance. I am 6'2" and ride a 19" frame, and for when you biff and have to jump, you want the cleaance so your "valuables" don't get crushed.