How do I introduce my new duck to my other ducks?

[Babbette3741]

When I first purchased my ducks I had two male and two female Welsh Harlequin's. I show them at the fair. About a week ago I had a male and a female at home and another set at the fair. One night I went to go put them into the coop and found my female had died. It was very unexpected, and I was extremely heart broken. We placed my extra male Pekin's in the yard with my drake until I could bring home the rest of his flock. He adjusted quickly to the two male pekins. A few days later we brought the last two Harlequin's home and my lonely drake rejoined his flock.
Yesterday I brought home 2 female pekins and one female buff duck to give to my two male pekins. All of the pekins are going to another home to be pets. The buff could go too, but I want to keep her to fix my ratio in my Harlequin flock.
How do I introduce the Buff to my Harlequin's? I have a smaller pen I can place them in during the day time, but they have to go back into their coops at night because we have a coon.
Should I put the Buff with my female or one of my males?
Should I move them out of sight of the other ducks so they think they only have the two of them?
What is too much aggression?
I could send the buff with the pekins, but I would rather keep her.

So just to make my number of ducks clear:

2 male pekins
2 female pekins
1 female Buff
2 male Welsh Harlequins
1 female Welsh Harlequins

I want to make a flock out of the WH and the Buff

Thanks!