7-9 week old ducklings are not ducklings anymore, they are loud and cheeky »ducknagers«. The girls should already be quacking.
How big is the "rest of the flock"? - If the numbers are equal, or you have more ducknagers than grown up ducks, i would just let them loose and they will "fight" it out quickly.
To merge a few ducknagers with a larger flock you must keep them separate for 1-2 weeks and observe their behavior. First the grown ups will be somewhat aggressive to the "intruders", but will get used to and in the end accept them. Well fed ducks with enough space are usually very welcoming to additional ducks after a short phase of disapproval.