African cichlids are fascinating fish. When introducing to a new aquarium, it is best to add as many as you can at one time. This will cut down on aggerssion issues. Also, re-arrange the decor in the tank when adding new fish. It will make the existing fish find new dominating places. This also cuts down on initial aggression.
When introducing new fish, feed the existing fish on one side of the tank as you are putting the new fish in the tank. The older fish will be slightly distracted with the food and won't pay attention to the new fish. The addition of the fish should be done around feeding time OR just don't feed them the day that you are adding the fish until they are introduced.