Your gunsmith can cut it to any length you want. Just about ALL of the custom barrel makers hand lap their barrels before they ship them, including Rock. Because of this it is recommended that at least 1" be cut off of every barrel to remove the portion of the barrel that gets flared slightly during the lapping process. As far as the twist being to fast for lighter bullets, depending on what cartridge you chamber it in, and the velocities you generate, it would probably not work well with Bergers with standard jackets, and possibly even their target jackets, but I think the Sierra Match Kings in the 200, 210, and 220gr should hold up fine.
Paul at Wildcat Bullets told me he hopes to be starting on the .308 cal 265gr RBBT bullets this summer, and he recommends and 8 twist for them.
Hope this helps.
Dave