If he's a gentleman, he might only need to be separated by a seethrough barrier in spring. I would get only females then, or like 6 or 7 females if you want a male too.
As @JacinLarkwell said, (and I mentioned on your other post) it depends on your drake. If he’s good to the ladies and doesn’t force himself on them, he may be fine, and only need to be separated if his hormones are really raging to give the girls a break and prevent injuries. The biggest issue with the size difference is that he’s much larger and heavier which means he has a greater chance of injuring them, or drowning them accidentally if he hops on in the water, not necessarily that he’s going to TRY to hurt them, he’s just a big dude. Definitely less of an issue if he’s the only drake though, as a lot of the problems with drownings come into play when a couple or a bunch of drakes try to pile on top of the same hen. You’ll just have to play it by ear and see how he is with them and adjust as needed. Alternatively, any chance of getting him ladies a little closer in size to him? That may help take some of the pressure off the mallard. although, if she’s his favorite, it won’t matter how many other girls you have.
Have to see how he is come spring. 
Cayuga are a few pounds lighter, but definately larger than malards. I think around 7 pounds for girls?As @JacinLarkwell said, (and I mentioned on your other post) it depends on your drake. If he’s good to the ladies and doesn’t force himself on them, he may be fine, and only need to be separated if his hormones are really raging to give the girls a break and prevent injuries. The biggest issue with the size difference is that he’s much larger and heavier which means he has a greater chance of injuring them, or drowning them accidentally if he hops on in the water, not necessarily that he’s going to TRY to hurt them, he’s just a big dude. Definitely less of an issue if he’s the only drake though, as a lot of the problems with drownings come into play when a couple or a bunch of drakes try to pile on top of the same hen. You’ll just have to play it by ear and see how he is with them and adjust as needed. Alternatively, any chance of getting him ladies a little closer in size to him? That may help take some of the pressure off the mallard. although, if she’s his favorite, it won’t matter how many other girls you have.
For sure, mediumweight breeds are better than lightweight or bantams in a case like this. I guess it also makes a difference if he’s a bit of a larger pekin or on the smaller side.
But the cayugas available around here are pathetically tiny, very poor hatchery stock, so it also might depend on what stock is available...
I think you actually have a hybrid white layer look up pictures of drakes they look just like pekins but they are much smaller.
