I agree it all depends on the new comers and the established flock.
Locally, three rescued Pekin females went to join an established flock of a pekin drake and female and their 2 adolescent ducklings. The adolescent ducklings took to the newcomers immediately -- their own parents virtually ignore them. The ducklings and newcomers are all friendly pekins! So that went well [yes the rescues should have been quarantined, but the rescuers didn't realise. Fortuantely all healthy!]
I just introduced a rescued young Pekin drake to my son's three females [2 pekins and 1 muscovy]. He and his brother have been with me since the beginning of May. All went well yesterday, on their first day, so he was allowed into the coop when it went dark. The girls gave him a bad time this morning shouting but I was there to let them out at 6am. They had a really good day today: he doesn't like the muscovy who keeps trying to be freindly but he moves away; the larger of the two pekins keeps making go away head movements towards him that he ignores; and by this evening the three ladies were following him in a line round the garden. The girls went into the coop first but he followed them and they all ate from the same bowl. That has worked out well, fortunately. We didn't need to have him sleep in a dogcrate in the coop, although we had the dogcrate at the ready.
Introducing his brother to my all drake flock is taking longer, as I have a pekin drake that "looks for trouble". The rescue has been in a pen with a kiddie pool on my patio for 3 weeks. My drake who looks for trouble, is going in the pen tomorrow, and the rescue will be free ranging. The rescue knows his way around as he alternated with his brother being in the pen or free ranging for the last 3 weeks. Its going to take time to get him fully integrated. The pekins sleep in their own dogcrates in my coop.
I think introductions generally do best if they can sleep in separate compartments or in dogcrates in the duck house initially.