I would separate with a view for at least a week. Little ones are easy to kill for even the one week older chicks. So the time is to let them get their legs and to make them "normal".
I am going to try to integrate with a three-week difference in about a week. Not sure how long I will just let them look at each other.
I know ducks and geese are different--I have both--but with my geese, this is what I did:
One goose was out alone (before any of their eggs hatched). Seeing her, I wondered what would happen if she were to see a gosling. I had eight in a brooder, so I brought one out and set it down. She saw it and immediately started hissing at me, "driving" me back.

She spread those big wings over the baby as if to claim him as her own, and started herding him away from me. I hurried and got two more, which greatly excited her. She did the same thing again.
She seemed to be herding them toward the coop. (It's a round-about route, so not for certain yet.) One of the other females came out to meet her and helped bring the babies in to where the third was sitting on eggs, watched over by the gander. They took my babies away from me. All of them. When their one successfully hatched gosling joined the group, and they brought everyone out after a day, I released the remaining five in their presence, too. All grafted and done.
I have another four (one hatched so far) I'm thinking about trying them with, but that really WILL be a big discrepancy. I'll probably keep them a week in the brooder (at least), then see whether they'll take them. They're amazing parents.