Nothing personal, but skip the Chromis and Yellow-tailed Blue Damsels. While the Damsels are colorful (I have one) and the Chromis are easy to keep (got two of those...used to have 7), there are other very easy to keep fish that you may enjoy much more.
But for a 20g, keep in mind that you'll only for a couple fish. Maybe 3 small ones....and by small I mean that their full grown size is only no more than 2 inches.
What about a Firefish? Perhaps a Gold Banded Maroon Clown? (yes, they can be mean but they are very good looking) Maybe a Yellow Assessor? Purple Pseudochromis? Six-Line Wrasse? (some will say that a six-line requires a pod population, but this is not necessarily true....many will take frozen foods...mine does and I know several others who have ones that do)
As to corals.....zooanthids, leathers such as cabbage, sinularia, pulsing sinularia, Kenya Tree for an "empty spot filler", etc