We raise ducks for eggs - our youngest kiddo is allergic to chicken eggs and we got tired of the “high end” grocery stores running out of duck eggs during the holidays .... every year.
Khaki Campbells are excellent layers, but also escape artists in my experience, because they fly so much better than the Welsh Harlequin ducks.
Our Welshies lay close to an egg a day. I do prefer to keep a drake, because he is very protective of his ladies and they are a good “watchdog”. But, the hens will lay without a drake involved, and some people prefer to have no drake / don’t want to have fertile eggs (although unwashed duck eggs that are laid in a clean area, stored pointy end down in a coolish spot, keep extremely well...)
I prefer the WH, because they can fly well enough to escape some predators, not well enough to be a pain to keep in their yard on days that I’m not home / they need to stay in, lay lots of eggs, and are big enough to use for meat if desired.
Muscovy is the next breed I’d like to try, as they are helpful w flies/ bugs/ mice- but aren’t the strongest layers.
I may be in the minority on the mixed flock part of your question; I’ve found that my ducks (and turkeys) tend to group up by breed. Yes, it can work, but my most “harmonious” flocks have been those similar in color and size. I know that sounds funny, but, I’ve seen it many times in my flocks. I’ve never had chickens, and I know that doesn’t seem to be an issue w chickens based on what I’ve read and what my chicken keeping friends have told me.... but my turkey and duck experience has led me to sticking w one variety per coop as much as possible.
If you are able to provide a larger yard attached to the 6x8 “coop”, you could absolutely go with more ducks.
If you are planning to use the a frame as a “tractor” and that will be their main space... I don’t think that would be sufficient, personally.
Because.
Ducks are very messy w their water. And. My understanding is that they need a pool to maintain their feathers (esp the oils on their feathers, which helps them manage temperature fluctuations and be more resistant to pests) properly (and for breeding, should you decide to get a drake)
I keep a couple of 3’ kiddie pools in my run, they are big enough for several ducks (probably about 1, 3’ pool for 6-7 ducks at the most), and not a huge pain to dump, clean, and re fill every other day or two.
I also have a 5gal bucket that I took a 2” hole saw to, made 2 holes (aligned vertically), and cut out in between the holes to make about 5 ovals that are about 4” tall x 2” wide. The lower side of the opening is about 5” above the bottom of the bucket. I clean that out and add water twice a day; it allows them to have fresh water if I haven’t had time to clean their pools out, is deep enough that they can dunk their face to clean their nares (nostrils), they can’t really poop in it lol- and it stays in the coop.
They have constant access to oyster shells and grit. I feed always in the coop. Twice a day. Crumbles, a bit of rolled oats (dry oatmeal), and a few ground up brewers yeast tablets (labeled for dogs, from my feed store, to add niacin to promote circulation and foot health)
Red cabbage and collard greens finely chopped and added to a dish w water are my ducks’ favorite treats.
We let them out to free range whenever we are home, and they don’t tend to stray far from their run (unlike the turkeys

)
And my ducks always are happy to come in for bedtime knowing that dinner is served in there.
Sorry for the long post. Just my 2c
