As a reef tank matures it developed its own natural clean up crew of mini snails, bristle worms, pods, brittle stars etc. In my 120gal system the only store-bought inverts I have are two abalone and a tiger tail cucumber. I would like to also have my tuxedo urchin in there but the overflow has a knack for killing them.
So in other words keep whatever you want. Just do your research to make sure that what you get won't starve to death or kill your other tank inhabitants. Abalone are the only snails I care for. I've tried at least 6 common species and none of them could hold a torch to abalone. I believe that crabs are more trouble than they're worth. And why bother with buying a big brittle star when you can easily have a million free little ones?
This picture is a bit misleading. There are five fish in the display, one of them being a +2' snowflake moray so this is also a high bioload system.
