They are good guards, but our polish rooster is actually the watchdog of the entire flock. He alerts everyone when a hawk is in the area and does his best to make sure everyone gets into cover. And with that said. Our alpha pair of runner ducks is the first to charge our dogs if they come anywhere near their area. Our German Shepherd went into the pen with me one day and they cornered her and were pulling hair, she looked at me and begged me to get her out. But ducklings can be hurt by chickens so they need to be grown, I wait until between 4 to 6 weeks to put them into a cage within their pen to introduce them, then release them around 8 weeks.
I've heard that the ducklings need to be about the same size when they are introduce to the chicken flock. Plus I'd be following an integration plan similar to what I'd need to do with new chickens. My babies' safety comes first; whether they're chickens or ducks. I might be new, but I'm going to go the most cautious route.