Clowns will get too big for a tank of that size, as will damsels. An anemone would be unnecessary and impossible. There are small goby species, in the genus gobiodon, that may be OK. Neon gobies are hardy and very small. The odds will be against you, though, starting with such a tiny tank. Very hard to keep such a small system stable.
I don't want to discourage you from going marine, but I have a couple of suggestions. First, buy or borrow a copy of Bob Fenner's Conscientious Marine Aquarist. It is more up to date than most marine aquarium books, and it will answer a lot of your questions. I bet you a buck that you'll be glad you read it. Second, buy a bigger tank. This is a case where bigger is better.
The sale of colorful nudibranchs should simply be banned. They are almost all very specialized for certain food species, mostly sponges, so they will simply starve to death in most tanks. Bill Rudman, one of the world's experts runs the
Sea Slug Forum, if you wan to learn a lot about these guys. He has a section on captive husbandry, and the upshot is that you shouldn't even try. There are a few species that will eat algae, but they aren't as pretty.
Hate to be a downer, but I always try to push people in the direction of long-term success. Stick with FW for the moment, and collect equipment for a marine tank as you go.
