Wow..... you're asking questions that are "way" out of my skill and knowledge levels, but I can give it my best guess with the first one.
The three axis laser ring gyro started to appear on the 737-300 during my very early days as a mechanic (1987).
I believe that all three gyros are exactly the same size and are interchangeable. The only thing different is how they are mounted. One sensing pitch, one sensing roll, and the final sensing yaw.
I was told that during the alignment period, the units actually sense earth's rotation. True??? I don't know, but at the time of their arrival (aircraft moving from analog to digital)..... they were considered pretty "whiz-bang" units.
