Ok, I will impart some of the (limited) knowledge that I have!! LOL
There are a number of posts on here about keeping chickens and ducks together. While some do it, for the most part, it seems it is not recommended. Ducks make a MESS with their water. I keep the food dish and water dish pretty far apart and it stays dry for the most part.
I firmly believe ducks needs swimming water, and a few bowls of clean drinking/dunking water every day. They are ducks, so swimming is inherent for the most part, and I feel to deprive them of it is just mean. And not fair. But they do need clean bowls of water, large enough to dunk faces in and drink from, but not big enough to swim in. One of mine is small and she will get into ANY pan of water she can. So I put a board across one of them and they can dunk and drink, but not climb in.
Drakes can hurt and kill hens if they mount them. Not anatomically compatible. Even if you get girls, again, probably better to let the chickens and ducks have their own spaces.
Which brings up another thing, make sure you have the right ratio of drakes to hens. One drake for at least 3, more is better hens. I guess you could do 2 hens, but my personal preference is to have at least 3. I have 5 girls and a boy and it works great.
I believe I read that ducks and chicks can eat the medicated starter food, but do some searching on here and make sure.
If you get ducks that can fly, yes, they will take off. Some will return, some not. You can clip wings to keep them home.
Ducks are great, messy, silly, noisy, but such fun. You will love them!!!!!