What size polishing pads

I like your idea to rough with steel bell reducers and real grit.  Unless you're grinding marble or softer stone, the plastic pads are kinda wimpy. They will eventually get the job done on softer rock, and they work just fine after roughing the sphere, and last a long time for the finishing grits. Polish with a plain buffing pad at 1 micron grit.

I started with 3" pads, making baseball & tennis ball sized spheres. Tennis balls are 2.5", and the 3" pads are nearly interfering with each other, A smaller sphere may fall through between the pads. 2" pads sound just right to me, especially since you've already rounded the rough sphere in steel cups. Your link to the pads looks like a good price for 7 pads and a backer. The backer may have 10 or 14mm threads. Verify the threads with a hardware store bolt.

Note that the 2" sphere touches the pads only near the center 1" or so. That's the area which will wear out first. The outside part of the pad is pretty much untouched. Cut a new center patch and just velcro it on. If you find a cheap price on 3" pads, just cut them down to 2" with old wire cutters (don't use the good scissors lol). Cutup pads lose their grit identifying numbers, so I keep each grit in a labeled zip-loc bag, because you can't tell the grit by eye.

I'm trying kitchen countertop hole saws, but they are only electroplated diamonds, and not much there.

Good luck with your machine, and please post pictures!

To make it easier braze diamond embeded brazing rods onto the bell cups.

And here is agood how to on brazing your cups.