The duck in this pic was about 2 or 3 weeks old when put in with 3 chicks.
She's still a chicken at heart nearly 6 months later..
The aggression was instantaneous with the duckling who wouldn't accept the chicks; if she's sharing a space without squabbling then there's no reason why they...
Ducks and ducklings don't like change but they're naturally curious so it won't take long till they get to know each other.
You may wind up with a duck who is more chicken than duck tho: but it's all good!
Edit - scared is a good start; I've tried mixing ducklings and chicks several times and...