Actually some corals like green star polyps grow fast enough to over take slower going ones. Kenya trees have a way of dropping branches and them growing all over, the same with xenia but xenia can move too and leave a trail of little corals. Toadstool is fast growing and easy but doesn't spread and easy to frag. LPS like frogspawn is easy to grow also.
Remember the type and amount of light will have a lot to do with the corals you choose.