Don't know if guineas and ducks get along or not.
I would put him separate but by the ducks for a week, maybe longer, until he is close to the same size as them.
Be observant and see how the ducks warm up to him.
Once it seems everyone is comfortable and he's similar in size so he can...
I'd keep him separate from the others at first.
If you can somehow make a pen for him next to the others so they can all start to get used to each other.