You do need to take into consideration that the corals and rocks in your tank will use up space that could be used for swimming room if you were a fish. Another words, a tank that is 100 gallons in capacity but stuffed full of rocks is not going to be able to handle twenty inches of fish. Corals also take up space in somewhat the same manner. The corals don't add to the bioload, but they do affect the tank capacity to hold fish.
im a freshie learning to make the jump to sw, but i had to comment, have you seen some of the decorations and plants we put in our tank. that would be like fw people adding caves and plants to their fish count.