I have a black spiny sea urchin and I love him! He's getting really big, but I 've never had any problems with him moving rocks....He did however, eat one of my feather dusters crown!!!!! Didn't know he could do that...he ate the whole thing (well, what he pulled off). He moves all around the tank, glass and rocks.
I also have a pushpin urchin that is super fast. He moves everywhere and the only problem I have with him is he likes to perch on the airator and moves it sometimes. Otherwise he's a joy to watch!!
They are both great additions.