20 gallon tanks should be limited to about 3 fish or so, depending on the occupants. The scooter blenny, as said above, requires a lot of LIVE fauna to eat, so make sure and keep on top of your tank with live supplements of copepods (unless of course, if you can get it to eat frozen food).
As for the maroon clown, shouldnt be a problem, assuming you wont add any other fish. Maroon clowns tend to be quite aggressive, and can become very territorial in such a small tank.
Look for small fish (start by going to liveaquaria.com, and check out the marine fish section under Nano Fish. Thats a good starting point)
I currently have a 20L, and kept a mated pair of percula clown fish and a rainfords goby in it. Hope that helps!