Right, ducks won't directly harm fish but as ducks like to get their beaks into all sorts of things they may put harmful bacteria into your pond.
Call ducks are nice and quite small, but bear in mind their name they are not exactly stealth ducks!!!
Other things to consider is are you ready for your garden to be wrecked??
Ducks like nothing more than dabbling in water then soil and making lots and lots of mud. Ducks poo whereever they go so be ready for dodging that all around your garden.
As fo general advice, there's one book I would heartily reccomend and it's this one -
Starting with Ducks: Amazon.co.uk: Katie Thear: Books