I've been reading the docs and I would like to add more decorative stone (like andesite) to the game. Are big blobs using the ore decoration the only real option? Is there a way to add new stones to the built in mix? Would it be too heavy to make the blobs after ore gen so that they don't hinder resources?
Thank you.

Your best best bet currently would be an ore decoration. Current stone variants do use the ore decoration, actually. They use a size of 33 and count of 10. It shouldn't impact your performance too much to add a few ores.