I'm glad you took the time to post today. I wish you had posted prior to purchasing any equipment or the aquarium, but it sounds like we are to late for that.
Lets start with the basics of marine aquarium care. We discuss this topic almost daily here, so it is easy to find this information in detail on these forums. Also, almost every magazine dedicated to marine aquarium care has this information easily available. I give you this disclosure up front, because there is something critical that you need to know and you need to know today. Ready? Ok, here goes...
The pet shop you spoke to provided you with information that is wrong. This happens a lot unfortunately, especially in the keeping of marine aquariums. There is one big reason why marine aquarium care is more difficult that freshwater care. The reason is that the pet shops cause this to occur by not properly educating themselves and their customers on the proper methods of caring for marine aquariums.
I have no idea how much knowledge you have of fishkeeping, but I have to make some basic assumptions in order to have a conversation. If you get lost in the following paragraphs, then you absolutely need to spend many more weeks researching this project.
The filtration system you have does 2 things very well. First, it traps waste so that the waste can be removed from the VISIBLE water column. It does not remove the waste from the water flow, so the waste is still present, it simply removes it from your viewing area. Second, the ceramic media and sponges grow bacteria which break down ammonia, producing Nitrite, and break down Nitrite, producing Nitrate. The biological process stops here. This is a wonderful system and very effective for FRESHWATER aquariums.
In a marine aquarium this system causes all sorts of problems that make long term success extremely difficult, hence the reputation of marine aquarium systems being difficult. First, lets talk about the filter pads and how effective they are at trapping waste. In a marine aquarium, ammonia is not directly released into the water by the livestock. Instead, organic acids are released. As these organics bond to surfaces, such as filter pads, they begin to break down into ammonia. In your system this process continues to produce nitrate.
The breaking down of organics causes a depletion of carbonate, which are a critical part of the buffer system. It is these carbonates (and bicarbonates) which make up the alkalinity of your aquarium. The long term success of a marine system depends highly on maintaining stable levels of alkalinity and calcium. (Calcium is a major element which bonds with carbonate and bicarbonate ions to buffer the water.) The biggest stressing factor on fish in a marine system is the instability of alkalinity levels, which leads to swings in pH. Your particular system makes this process very difficult to maintain.
Another drawback to mechanical filtration is the side effect of phosphates being produced. Phosphates cause major problems in marine aquariums, leading to massive hair and bubble algae outbreaks.
The next problem with your filtration system are the sponges and ceramic blocks. Because these media grow bacteria, the break down of organic waste is even faster. Sounds great? Not. This break down leads to Nitrates, which we are trying to keep at near zero in a marine environment. It is not a problem for a freshwater tank to run Nitrates well in excess of 100ppm, but in a marine system Nitrates are a stress factor that lead to disease. You NEED Nitrates under 40ppm and WANT Nitrates near zero. Your system intentionally ADD NITRATE TO THE WATER. I would love your LFS to explain this selection in filter systems.
The correct method for maintinaing a marine system is live rock, live sand, and a protein skimmer. The protein skimmer directly REMOVES organic waste from the aquarium. The organics are never broken down, saving the carbonate buffer system, and eliminating the production of nitrate that occurs from this biological process.
The live sand and live rock grow different forms of bacteria than you have possible with a man made biological filter system such as yours. These bacteria will also process organic waste, but this will be incidental, not forced flow. There is an adidtional bacteria that grows within the depth of the rock and sand. This bacteria breaks down NITRATE, producing nitrogen gas, which leaves the system naturally and is non-toxic.
If you spend time browsing these threads, especially the "Pictures and Videos" area of this website, your will find that almost every successful and experienced marine aquarist using this method.
Now, to answer your direct question. You have almost no chance of keeping a Yellow Tang, Purple Tang, Pacific Blue Tang, or Racoon Butterfly in an aquarium set up with a biological filter system. You also have issues with tank size, as your aquarium is far to small for the Purple Tang , Pacific Blue, or Racoon Butterfly. Marine fish need a lot of room to move, or their health deterioates quickly. Your choices of a Dottyback and Clownfish are much more appropriate.
Lets first get this system set up correctly. Then we can discuss fish in detail.