We have 8 hens and a rooster about 3 years old which we have raised from hatching. I think they're RI reds.
About a year ago we added a pair of buff orpingtons and a pair of australorps. These were initially timid but are fairly well integrated now.
About 3 weeks ago we added 4 leghorns into the flock. We'd already had them for about 5 weeks and kept them in the pen but fenced off from the others, so they could be seen but not reached.
I'm seeing 8 original hens, 1 rooster, 2 orpingtons, 2 australorps, 4 leghorns. So that's 17 total.
The big pen is about 5m x 4m.
That sounds big enough for 17 chickens, even if they were confined to it day & night. And you've said they have even more space in the daytime. (For those of us used to working with feet: it's more than 200 square feet.)