Ok well, there's a thread in this forum about the smallest fish that people keep in reef tanks. You might want to check that out.
IMO you should get either a high fin red banded goby or a yasa hashe goby, either of which will pair with a randall's pistol shrimp. That's a wicked awesome goby shrimp pair, no matter what size tank you have and they won't outgrow the tank...ever.
I also really like Lisa's suggestion of having some cool inverts in there. Inverts are super neat and very active.
You might also consider one green banded goby and maybe one clown goby OR one red head goby OR one firefish. I wouldn't put more than 3 of the fish that stay really small in there and CERTAINLY not a clown. I don't understand why everyone under the sun wants Nemos.