Oh yes, I think you've got a point there with the generic Princes. And there are a few generic Princesses too, I think - usually those whose rash fathers have promised their hand in marriage to the first-comer who can do X, Y and Z. (I remember in one of the Monty Python books there was a King who lost so many potential suitors that way that he asked the last Prince to go to the newsagent and buy him 20 Rothmans. I think the Prince failed anyway.)

But is Melisande (and therefore Florizel) a traditional fairy/folk tale, or just a creation of Nesbit's? In that case she probably just borrowed the name from Shakespeare.