I would wait till their larger/older.. an established flock of that many and especially with a drake? i would aim for a bit longer yes, sometimes it does work okay younger but being 1yo they are fully mature, mating and laying ducks now..
what breed? are they big babies or wee birds, same breeds or different.. do you know their sexes? all those play a role in the transition, for what it's worth i often continue to house new young birds separate till they have come into their own so closer to 3-4mths and even then some are still kept apart at night...
The biggest thing is ensure everyone has access to feed/water and plenty of room to get away from on another. Some birds are quite laid back and newbies drift in others have very tight pecking orders and dislike newcomers.
Have you ever introduced new ones to this flock? even young of their own? past reactions help you gauge as well. My main flock has seen quite a bit over the years, and many, many young that has given me insight to their acceptance overall of new ones.
My Motto, hope for the best, plan for the worst. ALways have a back up plan if it goes poorly.
GL!