To get baby yumas on small pieces of rock I'd use a dremel tool or a wet tile saw to cut the rock between the yumas if you can.
I've never tried the rubberband method for a yuma, but I've had something similar work pretty well for Ric floridas. I would think it would work for yumas too, but at higher risk, since yumas are more sensitive.
As for the best way... I don't think I'm experienced enough in that method to give sound advice, but I'd personally use a smaller gauge rubber band and try not to make it too tight to start with and maybe insert something (thin rock maybe?) underneath the rubberband on the other side of the rock as more tightness is needed to cut through the polyp.