Supergirl ought to be a badass woman, and for that I may only nominate Hollywood's resident Badass Woman, Michelle Rodriguez. At 36, she's not as young as you would prefer, though I personally would like to see a slightly older Supergirl.
You've got some perfectly good choices, but I'm gonna toss in a non-blonde option (b/c hair dye and wigs are perfectly usable...for example, Christina Hendricks isn't a redhead but she's extremely well-known with that look from "Mad Men") and go with Allison Scagliotti. She's currently in her early 20's which would be exactly the age range you've said they want for Kara, and on Warehouse 13 (her "biggest role" so far, as Claudia Donovan) she did a good job of balancing intelligence and enthusiasm with uncertainty and sarcasm, which seems like it'd be at least part of Supergirl's characterization. Of course, then the W13 fan in me would then want Saul Rubinek as a mentor character to Scag's Kara and Joanne Kelly as Wonder Woman (or some equivalent big-sister heroine to work with Kara). I guess I'm saying that I want more Warehouse 13. But also, Scags would probably be a good Kara.
Well yes, but I was using Christina as an example of an actress who can carry a good performance even when the character has a different hair color. The actual candidate I'm suggesting is Allison Scagliotti, who's of the right age, has experience performing as characters with similar personalities, and is probably a good physical match apart from not having blonde hair.