When asking about risk, you have to know what is the potential loss. How big is your chicken flock that will be exposed? To answer your question most safely, start with a list of every disease that is undesirable.
Fowl pox, Cocciodosis, Mareks are the ones I can think of. Then find out if ducks can be carriers. Ducks can carry Fowl pox and Cocci, but not Mareks.
Then look up the quarantine time for those diseases. There are threads where people say they do not quarantine for Corrid because it's easier to just treat the whole flock. That leaves Fowl Pox, and now it's a bit fuzzy what to do.
The incubation time for fowl pox is 14 days. You can look at your ducklings for signs. You can ask the owner about his flocks history with pox. He may not know what it is, but ask if he has seen any scabs on his birds. If you visit his farm look around at his birds faces for scabs. Pox is transmissible even when dormant, so you just have to take the chance but it is best to ask. You do not want fowl pox, but then again some threads say fowl pox is the easiest to treat. I disagree, as I lose some birds to it. But it will not wipe out your whole flock.
Based on this, and my experience, I would take the ducks and not quarantine except for a day or two, unless you see an actual physical illness present. I have never seen physical pox on a duck. You don't want to introduce any Fowl pox to your flock, but even if you did, it may not even kill a single bird.