Its tough to say. USUALLY polyps of similar genus can grow together without harm. So, two zooanthid species of the same genus will grow up to each other and just find a common border. Sometimes polyps will grow amongst each other and not compete as well. Still other times, larger polyps will smother their smaller cousins for light and food and will eventually take over. And lastly, sometimes polyps (usually of different genus') will engage in chemical warfare and kill each other. The "biggest sting" will win out over time. Its impossible to predict ALL interactions between species, but merely trends therein.
Most polyps have a lower-grade sting that is really only effective against some fleshy LPS and some acro species. Acros really only ever out-sting each other. Soft corals are usually pretty inert but can smother other corals with their enormous size. Mushrooms actually pack a pretty nasty sting to some corals, are neutral to others, and generally loose to tentacled LPS. Tentacled LPS generally are your kings of the hill. I've watched my hammer coral just digest a mushroom that landed near it over the course of a day...
