Chocolate chip, general and Red African stars are "Docile" but predatory and can cause real damage in an inappropriate tank. They are not suitable for a reef tank and may certainly eat any slow invert like a snail. THis is their natural prey, and it should be anticipated that they will eat a few. However, all take well to spot feeding which is a draw (but may not prevent them from eating snails, clams, etc).
Linckia and "reef safe" seastars are a special topic as their needs are very specific. Please do research on those ( a search will turn up plenty)
There is no biological distinction between brittle and serpentstars. This distinction is made in the trade only. The green brittlestar (I think this is what is meant by olive) is a known predator in the wild, and has caused trouble in captivity. However, I have multiple specimens with a sixline wrasse, and they've never bothered him (previously even in a 15g).
Other people have issues with other species of both brittle and serpentstars...this is simply a risk of keeping any animal. Often, they are the fall guy, getting the blame for holding the evidence (a dead fish). However, people forget that they were put in there to "Clean up." Regardless some will actively hunt food. All should be spot fed. But it is not fair to say that only serpentstars should be avoided, or only the green. I have heard and some point or another of nearly evey species in the trade implicated, though I believe most who keep them have no trouble (regardless of "brittle" or "serpent"). As mentioned, most reports implicate the green brittlestar.
Urchins are OK, though they bulldoze rockwork and the long spine is poisonous. You must always know where it is relative to your hand. The slate pencil urchin is not considered reef safe. Also note that they have a habit of grazing on coralline algae which may upset people who have tanks where it is slow to regrow.
There are situations were these animals could hurt each other, especially predatory stars eating other stars or urchins and most commonly snails. Some brittlestars, based on personality, mostly, may pose a threat to small fish and various shrimp. But all are simply doing what is instinctive.