Wait a second. You put Discus and Yellow Labs in there yahoo....which i would advise against. Yes discus (a mated pair) could be kept in a 20gal...but only a mated pair and it'd probably have to be a 20H.
All the other choices are good. Apistos are great choices, Blue Rams, Tang Shelldwellers are always an experience though they don't have the color like the Apistos or rams. They have the personality.
What would you like to see in the tank? Color? Want it planted? Etc Just trying to get an idea of where you would like to go with this tank.