I have 15 x 7 rally wheels, what depth is correct for the trim rings, outer edge to inner edge? I see there are a few sizes to choose from.

The "correct" depth is @2.5" - but good luck finding a used GM set in good shape...
The aftermarket rings are available in 2" or 3".
You might want to check with Wheel Vintiques in California to see what they have available - I got the last couple sets of wheels I bought (with rings) from them.