Welcome Tim, great to have you here!
You are correct that they will perform best when they have room to grow. Some of them will form mounds of plants that will still look good, but some of them will crowd each other to the point the plants won't reach their full potential any more. All depends on the look you're going for.
This little clump of Sempervivum arachnoideum (bryoides) is very happy all piled up!
