So if there’s no adjustment to the corner cut outs on the slates, just as you say, the shelves get shallower as the pocket mouth specs get tighter.
In my opinion though, that’s fine. As pockets get progressively tighter than 4-1/2 inches, you need to have the depth of the pocket shelves getting shallower, otherwise the pockets will just play ridiculously tough. In fact, once you get down to 4-1/4 and less, you really should alter the pocket facing angle as well, to be a little more forgiving.
As far as shimming the pockets to make them tighter, I think even Diamond feels that 3/16 inch facings can replace 1/8 inch facings to tighten a pocket by 1/8 inch, but nothing thicker than 3/16 inch facings should ever be used to tighten a pocket.