One of the things I love about plants in the rockeries is the way they grow to fill any available space. Given space they just offset until they are overflowing.
The echeveria elegans stream turned into a raging torrent.
Echeverias seem to do that, not knowing when the space is full, bursting out of the available space.
The alpines are more subtle, a stealth attack.
Once they have a foot hold, tumbling over to continue their take over.
Others sneaking along in a slow advance, sending out the next line of troops to get another foot hold.
The agaves just occupy the space.
Whatever the tactics, these cramped spaces have produced some of my favourite plants in the rockery.