Tomei buckets are 1mm thicker (as you've noticed) so that you can run tomei procams (or any other camshaft with a 30mm base circle- standard base circle is 32mm so the extra 1mm in the bucket closes the distance down to stock-like spec). To run Poncams with tomei buckets you'll need shims that will be 1mm thinner than what stock buckets would need. The only need to run tomei buckets is when using smaller base circle cams.
My advice would be to run stock buckets with the poncams however, so the shims aren't getting too thin.
Running 30mm base circle cams with standard buckets requires 1mm thicker shims to make up the extra clearance, and people find that doing this can mean you spit shims out under high rpm, so the tomei buckets are the solution to that issue by using stock thickness (ranged) shims.
Out if interest, JUN uses higher walled retainers to stop potential spitting out of shims when using cams with smaller base circles, and there's a a couple of different options. But they're pricey.