Two of my favorite corals I wish my angels would let me keep!
Just use a bit of sense went placing the galaxea. Yea, place it on the rocks. If possible, try to look for a nice flat piece of rock with no sharp points, etc, and something that can support the whole coral. It does need a very wide birth from other corals.
The plate should go in the sand, that's important, but perhaps more important is to find a 'low-risk' area. If a rock were to tumble onto the plate, it would spell the end. I've had even large snails fall off the glass onto the plate and kill it.