Are the other ducklings able to eat and drink without her picking on them? That is a big danger as well, not just her physically harming them. Sometimes when ducklings are being bullied when they try to eat they will stop eating and then they will go downhill. The fact that the bully duckling is so much older than the others is a concern to me. Watch them carefully and make sure that all the others are able to get over and eat/drink without being bothered.
I had a duckling I raised next to my two older ducks (they're a month older). When she was finally the same size as them I took away the divider fence so they could all be together. There was some displacement behavior when she got too close to them (them nipping at her and she would move away). I figured it was all ok because they never injured her at all, but then I noticed she seemed a little weaker than usual one day. I felt her keel and she was extremely thin. Even though I had multiple food and water stations in their yard, the older two had not been letting the younger one eat! I separated her again immediately and fed her up with corn, snails, kale, and pellets too of course. She gained weight and is fine now, but I still keep them separated by a fence until I can get her some of her own friends. Just wanted you to know what can possibly happen in these situations, even if the bully/bullies are not causing physical injuries.
