THE biggest problem is if you don't know your birds, and what i mean by that is whether they have been introduced to birds in the past so you know who is are troublesome ones and who is your whatever, new duck.. moving on.
I am fortunate i have done this before, so i do know, fwiw, i have mama duck loose in the main part of the barn with her two 3wk olds so that means they mingle with 7 mature ducks and 1 drake, they also free range with the flock and no mama duck is not there 24/7 and when their loose that load increases since my 3mth olds(4 of them) are out there too, plus 1 more drake(16 birds total).
In my experience personality plays a massive role in this, if it's a tight knit group, there could be trouble, being girls(lovely btw) the drakes will be more interested your females may not, it's funny everyone always thinks it's the drakes but often i find the females are the ones with the issues they have a pecking order. For instance my young duck chooses to go with her brothers right now vs staying in the main barn with the 8 females and 1 drake, since the females jab at her right now, she'll come into her own soon but i find that is closer to 4mths vs 2-3.
Since you've always been doing the see each other through the fence game, i would get two people some sticks, and try a mingle and see how they react BE PREPARED to step in and fast if things go badly, i would observe the behaviour and study the reactions from each bird, and while this may sound completely opposite don't step in if there is some pecking, grabbing this is NORMAL and happens even in established flocks, it's important they realize their place in the flock but there is a point obviously where harm cannot be tolerated.
It never is easy, i love when i can raise the babies out in the flock transitions go better, i have 4 very tame ducks i need to bring into this flock of 16 and i am sorta not looking forward to it but being only 3-4wks they are much to young, for now they see the adults as they come by during their outdoor time lol