Agreed. I'd attach them somehow. Silicone personally.
I like blue painted backgrounds. Black looks great but blue seems to lighten it up and put me in a good mood. Black only look good with grey/silver or whitefish IMO.
EDIT: Ideas....Most people who have rocks stacked like that always get African Cichlids. I'd get more, maybe a few that get some size on them so it's not just a random talk of all the same size cichlids and nothing really to look at it.