There was a thread not too long ago where someone had similar scale issues and after much discussion and rot-fear, it turned out to be just some misshapen scales from curling up in a small hide. Full disclosure: i have never had to deal with rot, but your pics don't look like rot, just some darker scales - i imagine that on a white snake every little thing looks magnified. Obviously keep an eye on it and keep her quarantined as you would any new addition, but i honestly don't think you have anything to worry about.

Different topic: Are you sure she's a she? in the second picture of your original post it looks like i see some noticeable spurs by the vent, or am i crazy?

Oh yeah, and go sit in the corner for snake shopping at Petsmart...

(my Petco never has anything but baby normals, so i'm safe )