Hi. I am semi-new to this whole thing, so I'm going to try and give you whatever advice I know to be true. And from personal experience....
with regards to fish...just be smart about it. You can put whatever fish you want in there. Just do your research and see how fast the fish are going to grow. Obviously when they get too big I would move them out, but a juvenile clownfish would be more than ok in a tank that size. They take several months to grow to a size that is too big for that tank. Just think about it from a common sense standpoint....you can put any fish in any size environment, as long as it is the correct size to survive!
In regards to your ammonia, do some research on your own about cycling an aquarium. Don't change your water yet, let the cycle go through its progression and let the ammonia "live" in the aquarium so you can begin a proper cycle, and then when your ammonia goes back down (after a couple days in a small aquarium such as that) you know you are golden, and have enough algae or bacteria to handle your ecosystem....THEN I would change to clean water (avoid using tap water at all costs, reef hobbyist, or aquarists, will kill you if you use tap water).
Having a small tank like that, I would personally suggest stockpiling 10 gallons of RO/DI water form a local fish store (LFS) and 10 gallons of real salt water from petco (comes in 5 gallons boxes). That is the safest method to do water changes in a small aquarium!
Hope that helped!
