They only extend tentacles when feeding and they aren't very long. I wouldn't worry about them stinging nearby corals. I wouldn't suggest a black sun coral as your first as they're the most difficult (although my favorite as well) because they're harder and slower growing. Your typical Tubastrea yellow or orange sun coral isn't difficult provided you feed them daily or once every two days with my sis. They might not want to feed initially but after you start getting a feeding response they get used to it fairly quickly.Do they have any sweepers or just feeding arms?