I like
Catherine and most forms of it, too, especially
Catrin.
Kit and
Kitty are probably my favourite nns too - lol, if BtN was the real world, Kittys would probably be more popular than Katies! I also like plain, simple Cat, don't mind
Katya /
Katinka and even
Cathy would sound fresher than
Kate or
Katie, which are just so overused.
As others have said, going by your middle name causes confusion in official situations. My sister has never used her first name and, as she has a very common last name, has often accidentally thrown out letters addressed to R. Lastname because she doesn't immediately think they're for her! So I'd give her the first name you're going to use.