Well.. not really
Franciska, just a girl
Frances. When I was growing up, a lot of nicknames for girls were -ka.
Dominique ->
MikaMarianne ->
Anka Francis -> Ciska
But only for girls, so ^_^
Okay, so it's no-good for alliteration. That's okay. I don't think
Finn for
Frances is odd at all really, though I suspect some people might. You make good points about
William ->
Bill.
Of course in the end it's your choice but it's good to get an idea of what might happen :)