September is a GP of mine and I would liek to see it used, though it would be really strange to see it used on a baby born in June. Finn is okay as a nn. Finn is all boy to me though I can tolerate it as a nn for Fiona or Finola
I don't mind Finn on a girl, although I prefer it as a nn for Finlay or Finola for a girl & by itself or as nn for Finnegan for a boy.It feels fresh, light, and simple without being boring. September is not my style, it seems awkward & cumbersome. I don't mind May, June, January, or August though.
I have always loved September, but I think it is more of a GP. Finn is a very popular boys name where I live, along with Finlay, so I can't really get past that. I think it could be a cute nn for a girl, especially a tomboy.