You definitely can do more fish in that tank, but as said by everyone else, I would do the live rock first. It is in my opinion, and I am sure most will agree with me here, every fish no matter what it is appreciates live rock, not only for the biological purposes of filtering your water, but also for the natural hiding spaces, caves, protection, etc. You can name any fish and I could tell you how that fish uses live rock to its benefit. Take a trigger for example. You will find many triggers that like to protect themselves at night by wedging themselves under a piece of live rock. I don't think you can find too many ornaments that will provide a trigger with that type of protection. Then, you have fish like puffers. Live rock is great for puffers to get food off of when it floats to the bottom. Plus, puffers can grind their teeth on the rock to keep them filed. Then, you have the whole class of herbivores, like what you have in your tank. Every fish in your tank right now (with the exception of the clown) is a grazer and will definitely benefit from being able to eat the algae on the rock as they swim around in the tank all day. This is a whole lot more natural than using lettuce clips to feed your fish. Tangs eat all day as they swim in and out of the reefs. They do not just go in one place to eat.
I would definitly do the live rock. I feel it is one of the most needed things in every saltwater tank, no matter how large or small. I think it has MUCH more of an importance than a UV sterilizer. Yes, the UV sterilizers keep the water clear, kill free-floating parasites, etc., however, live rock has much more important and better purposes. Live rock serves to hold your biological filtration better than any media, denitrates water, provides fish with natural enviroment, and keeps your tank more stable. It will make the tank look a lot more full of life, plus it will serve every purpose that I mentioned above. Right now, your tank looks empty. No offense, don't get me wrong, the fish are very impressive, however, the full tank shot though makes the tank look like there is nothing in it. If you add liverock, it will basically look like a coral reef would look, just without the coral. We are trying to replicate the ocean in our homes. Live rock is the natural filtering process of the ocean and reefs. It would be silly not to use it in a tank if the purpose of a tank is to replicate the ocean.
Beautiful looking fish, none-the-less! I am really not trying to attack you, this is just something that I am adamant on as I feel it is very important. Keep us posted with pics and on what you decide to do with the tank though!