I love
James, and though I don't think it needs a nn, I'm fine with
Jamie.
Jim is a bit abrupt, but one of the nicest cats I've ever had was a
Jimmy, so I'm fine with that too. Unlike many less classic popular names,
James has earned its position IMO.
James on a girl should be too silly to mention. Especially since the delightful
Jemima is waiting in the wings ... I know there's no connection. but most people don't, and they really do sound similar enough.