if you want it to look like a mini reef (like bonsai) then you will have to constantly trim. Otherwise the best way on a tank that size is to pick one or 2 of your favorite corals and let them grow out. One coral in the wild will more than fill that space.
First, decide if you want softies or SPS. If it is softies you have a bit more flexibility in mixing and letting them encroach on each other. But this is a critical decision. If you want SPS then I recommend not mixing other corals in with them. SPS only tanks tend to be easier to maintain optimum conditions than to try to please the SPS and LPS/softies. Do a google search for reef photos or look in books at the different reefs. It is incredibly hard to mimic a real reef in a tank, let alone a relatively small one. To get the flow and light to hit all the areas of the tank, keep the water clean and limit the chemical warfare will be a large challenge.
All this said, figure out what you want to have in the end. Keep that vision and place corals with the intent to have them grow. Understand the growth rates so that you can plan ahead a little to avoid constantly reaching into the tank to move things.
Good luck!