The Copperband Butterfly will get too big for a 20L. Either of the angels are iffy, as they would appreciate more swimming room, but more importantly get most of their nutrition from live rock, so you would need to make sure you have enough LR balanced with enough swimming room to support one.
If you are dead set on a dwarf angel, take a look at Centropyge argi (Pygmy or Cherub Angel). They stay smaller than all other Centropyge angels, are very colorful, and full of personality.