Sometimes the biggest one isn't the top of the pecking order. I have several smaller ducks that are super feisty and will dominate bigger ducks. Often size is less as important than attitude.
Ruthhope is correct about the treats.
The spacing is definitely plenty. The only thing I would be worried about is the scraps and veggies. When ducks are growing they need a lot of protein and too many treats can lower that protein. Any thing that is not their feed is considered a treat. They are more likely to eat feathers if they...