Aggressive coral usually means that it has longer sweeper tentacles then most coral. I would give an aggressive coral 5-6 inches of room between it and any other coral. there are a few coral that will need even more room then that. Coral will not hurt fish with the exception of a very very few. Only two i can think of are the elephant ear mushroom (never seen one in a LFS
) and a carpet anemone (which you should not get until you have quite a bit of experience). Most coral if they are healthy will continue to grow indefinitely. You can trim then back in most cases if they get to large.