You are quick!
Thanks ...
As for the thickness of the string, you'll need to take some of the beads you're getting to a craft store, or jewelry making store, to just try the different thicknesses of string. If they're coming from a lapidary, and are tumbled stones, the holes might be bigger than they'd be if you were getting smaller beads from a jewelry supply company.
I saw some Rosaries for sale on a website that were 'Birthday' Rosaries. Each was made using beads in the stone color of each birth month, and the first few beads, and the bead in between each decade, were in varied shapes, just to make it more interesting, but were basically the same color. They were lovely!