What kind of cichlids? It would be safe with most rift valley cichlids, which thrive in the higher pH and hardness that the limestone will cause, but not as much with most river cichlids or new world species.
Limestone is a decent addition to any tank that requires a high pH and a high KH. It will also produce a high TDS. African rift lake fish are fish that do well in just such conditions but the same cannot be said of all cichlids in general. Which fish are you trying to get set up?
Yellow labs are African rift lake fish. Such fish, and simi9lar fish, can indeed benefit from some calcium carbonate in any of its forms, including limestone.