Hi! I'm new to the site and new to chicks as well! Right now I have a brooder with three Americaunas, and three Silver laced Wyandotes, (all 3-4 weeks old, can't remember when we got them!) another brooder with 11 newborn bantams.
In my outside coop, I have 6 one month old ducklings, (Pekins) and in the same coop, two small production reds, a Barred Rock Plymouth, a very large (we call it the Hulk) ISA Brown, and we just tossed in 6 three or four week old Leghorns. They are in an outdoor coop, 6x8 floor area, about 4 ft tall, hoop coop.
I think, in my experience, as long as they have enough space and enough feeders and waterers, they will all be fine. Ours seem to "self segregate" with on occasion the tiny Leghorns cuddling up to the huge ducks (I guess because they are all fuzzy and yellow?? lol) But mostly each group keeps to themselves. So the only time I wouldn't recommend mixing really large with really small is in a small space where they can't get away from each other, or in the case of really aggressive chicks.
In our Bantam brooder, a few are very very small, and yet it is one of the larger babies that is getting pecked at. We have done some things to try to discourage this, giving them things to scratch and peck at to eat, and making sure there are hiding places here and there.
Outdoor coop has a one gallon waterer, a 3 and a half gallon waterer, and two galvanized long feeders with the holes. I make sure that there is a feeder and waterer on each end of the coop. Then scratch is tossed down for the larger chickens. When I dump the old food from the feeders, I put it in a pile to keep the ducks occupied (they can and will eat anything!!) and then the chicks can eat without being bullied by ducks.
Also, they are all let out when we are home, sitting next to them. They are beginning to go farther and farther from the coop, but not by much. They stay very nearby!! The ducks swim in a big pool everyday. So far the Leghorns will not venture out of the coop, even with the door wide open for an hour. It seems they know they are not big enough yet!!
Have lots of cats also and two donkeys, all who leave the birds alone. As a matter of fact, I'm pretty sure they are ALL terrified of the ducks, which is hilarious to watch.
Hope this helps you....We are also being super careful with our Bantams, making mash feed, adding a bit of honey to it, medicated feed, little organic apple cider in the water...they are a little fragile as newborns, and we've lost two....so upped the care a lot. (as compared to all our other birds)
Good luck with your new babies!!! (As you can see, I could talk chicken all day....having a BLAST taking care of all these new babies!!!)
Many Blessings!!!
PeepsNQuacks (Jennifer)