just my 2¢, but I would wait until they get a lot bigger before putting them in with that pekin... with that kind of size difference, even if the pekin wasn't being aggressive, it could hurt them without even trying too...
This kind of thing can happen. One thing that can help immensely is making sure there is enough elbow room in the brooder. At first, about a square foot per duck. Then as they grow, the brooder needs to grow, too. Too small a space leads to aggression, feather eating, anxiety, failure to thrive, or other less than ideal behavior.
Try a larger brooder, and set a divider fence using plastic poultry fence or similar material, so that the ducklings can see each other and get accustomed to each other. The littles will catch up in a matter of weeks, and with supervised time together, they should bond into a little flock. Once the babies are old enough and have had some grit, let them enjoy pea treats at the same time, then together, and that will likely ease the transition into one group.
Yeah, the little ones cannot defend themselves right now. Once they're all closer to the same size, and can live in a MUCH larger area, you can try introducing them through a divider fence, feeding them closer and closer together until they can hang out right by each other (but still separated by fence) without getting upset.
At the moment, older duckling is just trying to defend his food and space, but it isn't a fair fight.