I would go with Taylor Momsen for Supergirl.
Taylor Michel Momsen (born July 26, 1993) is an American actress, musician, and model. She portrays the character Jenny Humphrey on The CW television series Gossip Girl. She is also known for her role as Cindy Lou Who in How the Grinch Stole Christmas, and for fronting the rock band, The Pretty Reckless.
She is young, very lovely and popular...soon to be even more so.