Length of shock is one key measure, the other is stroke, ie how far the unit moves under compression. Too little stroke will give less travel, too much and things may go crunch. Also consider that a progressive spring rate will have other ride influencing effects if the stroke is out of kilter for the frame.
Mountings... In my experience they can be interchangeable, they suit the frame and the shocks are fairly standard. Might be wrong on that one, I'm not that familiar with full sus either!