Can definitely go shorter on the jib board.
I'm 5'8" 180 lbs size 12s
My all-mountain is a Burton TWC Standard from last year. It's a 156W.
I'm riding the 153 Lobster. Very playful, loose and lots of pop.
Do you think that the 151 would be too short/unstable for you?
My all mountain board is the 154 parkboard at the moment.