Well, I have about 100 daughter colonies all over my 300 reef. They attach to everything.
Rubberbands do work, but almost always they WILL cut through. An idea is to put the frag in a depression in the rock, and rubberband over that, so when the rubberband is "tight", that depression in the rock prohibits it from cutting all the way through.
I prefer to stick a plastic toothpick through the base, then rubberband the toothpick to the rubble.