Keep in mind that salt water does not hold as much oxygen as fresh water, so a fully stocked salt water tank has a lot fewer fish in it than a fully stocked fresh water tank.
Two full grown True Percula clownfish (Like the ones in Nemo) will be quite snug in a 20 gallon tank. A single maroon would be too large at his full grown size. If you mean an Ocellaris variety by "black and white" you could keep one, with no other fish. Damsels are very hardy, good for beginners, but they are very aggressive, and should only be kept with other damsels.
You could consider adding shrimp and crabs and snails as colorful additions to the fish. You could also invest in live rock, which can have all kinds of interesting things living on/in it.
Other fish to consider: firefish, jawfish, neon blue gobies, yellow watchman goby...
Check out
http://www.liveaquaria.com/product/scateg.cfm?pCatId=2124, and pick out one or two fish that total 4" of adult grown fish. You get a little wiggle room with the pair of clownfish, the male will never reach full size, so two True Percula will fit although snugly.
Consider Shrimp:
http://www.liveaquaria.com/product/scateg.cfm?pCatId=525
Snails (but not conches):
http://www.liveaquaria.com/product/scateg.cfm?pCatId=526
and Dwarf Crabs:
http://www.liveaquaria.com/product/scateg.cfm?pCatId=501
Live Aquaria is a commercial site, but it has fairly accurate species information. Many other sites only list the size of the fish at the time of sale, not how large you should expect them to grow.
What method did you use to cycle your tank? (I congratulate you on not doing it with fish!)
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