RE: What are the most commonly used ply
What about balsa ply? I've replaced thicker balsa sheet components in kits with equivalent thicknesses of 3-ply balsa that I made myself. For example, sheet rudders can be stiffened by using balsa ply and they will resist warping.
I don't use lite ply because it really isn't very strong. A home-made laminate of balsa and thin birch ply is lighter and stronger. And birch ply is stronger when strength is needed.
I have bought 1/32, 1/16 and 1/8 birch ply, on the principle that you can make 5/32, 3/16 and 1/4 as needed from these.