run the shorter rafter section first, unless you are running a flatter section like a porch then run the flatter section first. you will run all the way up on the first side lapping a min of 6 inches...
It depends on the type of roofing material. It is best to put them in the ribs but it is acceptable to put them in the flats, usually on 18" centers along each rib.
Yes, a gambrel roof is the same as a barn roof. It is similar to a mansard roof, however the mansard roof has "hipped" corners and the gambrel roof does not.Hope that helps!