I had ducks, four. 4 weeks later, I bought four more, same breed. The first four were in the outside hen house and run 24/7 at about 5 weeks (warm climate). I tried to merge the flocks about six weeks later (oldest 11 weeks, youngest 6 weeks) after some days they spent outside together, but separated by fencing. They seemed fine - and remember, they had brooded in the same space (separated by fence) for about a week.
Things looked good, so I let them overnight together, thinking dusk and later, they wouldn't be active anyways.
The next morning, I was moving one of my young ducks - a male - back into the grow out box to recover from the beating he took from the larger birds. The girls took a beating too, if not so badly - and the elders weren't sexually mature yet, or I'd likely have had a drowning instead, as they have a small, but deep, pond in the run.
Supervised at 8 weeks, not in the water together. Try merging after 12-14 weeks, your girls should have enough size at that point to suffer the attention they will get.
/edit you will notice from my sig below that I now have 7 ducks, not the 8 described above. The 8th did mostly recover from his injuries, and was successfully re-integrated. And when a red tailed hawk (I think, my eyes aren't what they used to be) decided to prey on the flock while it free ranged, well... you can guess which duck the rest of the flock abandoned to his fate.