Introducing new ducks ???

Maybe, maybe not. What is the size difference?

My general outline is this:

Let them see each other from opposite sides of a fence.
Have a party with treats for everyone at the same time.
Let them meet each other under supervision in a spacious area.
Give them a break from each other ,
Repeat the previous two steps till they seem very relaxed around each other.
Note - if there is anything more than chasing and a little poking, separate and try again later.
Once they are comfortable together, let them spend a while together supervised.
Follow your gut - remember that sometimes being in close quarters, like in a coop at night, behaviors can change and there can be conflict.
Eventually most ducks learn to get along fine.
Watch out for any evidence of feather pulling or injury (I have had ducks pull leg muscles shoving each other).