I love both l4d 1 and l4d 2 characters, I prefer Rochelle over Zoey though, she seems more serious than Zoey, the way she handles herself and the entire "zombie apocalypse" situation. And they developed her in the "Sacrifice" - there she defended Ellis, from Nick's attacks about Ellis' love interest in Zoey.
Both of them seem pretty independent as people, and intelligent although this pretty much very abstract since they are infact characters in a video game. But anyway. And for the stereotype-thing I believe that people stereotype so much of reality these day, that they cannot say what is and what isn't a stereotype anymore. It's best to accept the characters in L4D 2 the way they are. Besides 50% of them are not from the South! I think Rochelle was from Arkansas or something like that, and Nick's accent is not southern at all, actually he has not got any accent, he could be from Chicago for all we know.