LOVE the old (and old looking) pics!
If I let one brood, do I need a separate place for her? Can she stay in the coop with the rest? I have another shed that I think housed birds or rabbits once that could be repaired and put back in commission for that purpose, and it would have a run attached to the same section of run that we have now.
I think most people recommend separating a broody, at least while she is setting on eggs and during the hatch. Other hens can lay in her nest or steal her eggs, she can steal others' eggs, she can return to the wrong nest, the eggs get jostled more, another hen can push her off her nest -- etc., etc. All she needs while setting is a small space to get off her nest and get a little exercise, maybe dust bathe, eat and drink of course, etc. I have done it both ways, lost some eggs, and did a lot of supervising. Now I separate during setting. Your Cochins, by the way, are probably your most likely broodies. You could always stick a Leghorn egg or two under your broody anyway, on the chance you're missing the bullseye.
I'm not familiar with most of your particular set of breeds -- but people let breeds mate other breeds and raise mutts all the time, including me.
I like to let mama and chicks in with the flock when the chicks are 2 or 3 days old, whenever they show signs of wanting to be with the others. A good mama will see to their needs and protect them. This way, when mama stops mothering (varies a lot, but probably averages around 5 weeks) the chicks are already integrated into the flock. Mine always kept to themselves for some time after that, but the hens didn't harass them, either. But a lot of people keep mama and chicks separate til mama rejects them. I've never had a roo or another hen do a chick any harm, but it can certainly happen -- and I've only raised 6 or 7 broods.