Not reef safe:
The green brittle star (ophiarachana incrassata) :
https://www.saltwaterfish.com/Green-Brittle-Starfish_p_385.html
Most starfish with raised bumps will predate on corals.
Mantis/pistol shrimps. Camelback shrimps (Rhynchocinetes sp.)
Banded coral shrimp (stenopus sp.) & yellow coral shrimp - eats snails - the good type snails & may feed on hermits crabs too.
Most hawk fishes will eat cleaner&ornamental shrimps & some crabs.
Sally lightfoot crab - will prey on corals & anemones when grown up.
That's what comes to mind at the moment.
The smaller the tank - the MORE research needs to be done before mixing things together cause there's less room for creatures to hide from predators.......and corals play biological warefare to begin with anyway - so much research must be done prior to throwing "what looks good" in a tank less than 30 gallons. And the smaller the tank - the more fatal it can become for species used to living in area the size of your neighborhood. . .