in the wild corals naturally crowd each other. they grow wherever they can. and like everything else, the dominant species will normally win out in a given territory.
there are places where you will many different species right next to each other, and places where you will see a single type dominate a given area.
additionally there those that have seen increased coral growth when the corals are placed closer to each other; based on the chemicals released by corals, this can stimulate growth as they race to grow faster than those around them.