I have gravel and it works fine. Crushed coral is recommended but either one will work. You will want to add some rock work and set it up so that there are caves... You also want to put the rocks in to help keep the PH up....
That depends on the kind of rocks you are going to use. Texas Holey Rocks and reef base rocks, will help to keep the pH up; Slate rocks and round river rocks for example, will not help on that.