I have a 10 gallon tank with a Percula Clown and a Bicolor Angelfish, and they are doing great together. My LFS told me that I could safely keep 3 or 4 Clowns in this size tank, so I guess you could have a pair or more in a 20gal. If you get other fish besides the Clown, make sure that they will be compatible together in a small tank. You don't want to spend your money on two beautiful fish only to have one of them end up killed!
As for the live rock question, I started my 10gal with 10lbs of live sand, 5lbs of live rock, and 3 damsels, and it cycled in a week. However, my rock was already cured, and the sand was from my LFS, so I don't know if you'd have the same results. If you want to spend the $$$, you can try this technique. It was sure better than waiting for a month while watching cocktail shrimp rot at the bottom of my empty tank. JMO though. Hope this helps!