Can gunite be added to reduce the interior depth of your pool?

A gunite pool shell IS waterproof before plastering.

First, gunite/shotcrete is not water proof. The reason for the plaster is to make the pool water proof. You don't have to and should not water proof gunite because it will affect the bonding properties for the plaster.

Contrary to the previous answer, Gunite is in fact "waterProof" when correctly applied. Unless there are improperly prep'd cold-joints (the point at which one application has stopped for some reason -perhaps rain, slow truck turnaround, etc- and begun at a later time), a Gunite pool could virtually be "painted", providing a high steel-trowel finish was applied to the Gunite. But, Gunite pops (begins to set and cure) faster than typically appied Marblite or Diamond Bright type cementitious finishes. Actually, the Gunite "finish" surface is purposely left fairly rouch, providing a better surface to which the finish/color material may bond.