I like
Quincy on a girl, too. More than
Quinn, anyway, although
Quinn is okay and could be used as a nickname.
It's odd because I have all male associations for Quincy--John Adams,
Quincy Jones,
Quincy, M.D., the little boy from
The Little Einsteins, and more--and both male and female assocations for
Quinn, but
Quincy still manages to feel more feminine to me than
Quinn.
It would definitely work on a girl in the US since just about any surname is fair game these days. I'm surprised that it
isn't showing up more on girls, actually.