Doing a little research before you got the ducklings would have told you that you can not add new ducklings to a group of adult ducks, or even bigger ducklings, until the babies are nearly full grown.
Can't you make a partition in the duck enclosure you have already, and have a section just for the new ducklings?
That would be the ideal thing, as the big ducks can then see and get used to the smaller ones as they grow up through a fence, so when you do introduce them to mix with the other ducks they should integrate with the flock much easier.
