No you did not screw it all up. First rule of thumb....do not panic and second rule work with it.
It is done now so we go from here. If you are worried about the little one, you could put a wire wall in the brooder so they can see each other and be near each other but the bigger ones cannot run over the little one. Or you could give little one a feather duster for a buddy...or a stuffed toy that would baby safe.
How are they doing together? Is little one doing okay stayed out from under their feet?
In future...never mix new birds with your existing ones without a guarantine of at least 30 days. Too many people have lost their flocks doing so and it is not worth the heartache. Ducks are not a bad about that problem as chickens apparently but might as well get in a good habit from the start. Plus, the difference in ages can be a big deal, especially with chickens...biggers ones will kill little ones. Ducks are not that bad but they do run over each other a lot and the little one could get hurt. Just watch them and if you feel they need some space, make a chicken wire wall and put it between them until little one is a bit bigger. She will be able to see and hear the others ones and not be as lonely.
Never worry about being in over your head. We learn as we go and we get better at it. This is my first time with ducklings since I was a kid and I have 11 of them in my bathroom. hahahaha
I did the same thing when I added 5 chicks to my existing 6 they were 5 weeks apart but were in the same room and not guarantined. I was lucky no one was ill. So, concentrate on finding a solution to the dilemma now and not worry about the separation.