Originally Posted by 55/Rad
You can use the short cage with the 12/27. Anything bigger than 27 and you'll likely need a longer cage.
Both the short cage and long cage are rated to handle up to a 27t cog. The difference is in chain wrap. The long cage takes up the slack from running a small (30t) chainring on a triple.
Good luck on the build grahny, and post some pics!