Introducing Ducklings to Chickens

How/when should we introduce ducklings to chickens?
We have 2 ducklings, pippa (golden 300) and poppy (rouen) that we got from tractor supply a little over three weeks ago. they were tiny, most likely less than a week old but were not sure. we estimate they’re 4-5.5 weeks old right now.
They’ve been spending their days outside in a little pen attached to a dog house for about a week now but still spend the night inside in their brooder tub because we don’t have a proper door on the dog house.
We plan on moving them so they can stay in the same duck accommodated coop and pen with our 7 hens and 1 rooster when they are ready.
What is the best way to introduce ducklings to chickens? when should we do that? anything we should be careful of?