I had a glorious time doing this show about 5 years ago. I played Ugly's snotty duckling sibling Downy. It was like my first principle ever.
Anyways, the cat is an amazing part. I think generally it is played by a boy. I've never heard of it played by a girl. And in my show he spoke with a french accent.
As for his vocal range, I think you can check the show out on this site, and it will say his vocal range.
Hope i helpd