- If I can get a broody to adopt day olds, how few can I get away with sticking under her?
The risk is not so much when the broody hen is taking care of them but after she weans them and they are making their own way with the flock. A lone chick can have trouble with that because it will be so lonely when the hen has rejoined the flock and the flock does not want the chick around. My suggestion would be a minimum of three so even if you lose one they still have a companion.
Can I stick chicks from TSC or somewhere similar, or would that be risking biosecurity issues?
Anything you do is a biosecurity risk, even if you don't go around other chickens. But somethings are a lot less risky than others. If the chicks in a feed store like
TSC come from a hatchery like practically all of them do I'd consider them very safe. If one of your neighbors hatches them, not so much.
Would it be possible to get bantams, or is that an integration nightmare waiting to happen?
People do it all the time. If the hen raises them with the flock and takes care of integration for you your chances improve even more. Is it possible you will have serious integration issues? Of course, that can happen if they are the same breed and color. You do not get guarantees with living animals and behaviors, it just doesn't work that way. There are plenty of cases on here where bantams dominate full sized fowl when they are mature, size isn't as important as the spirit of the chickens. The odds of problems probably goes up if they have weird feathers but if they are raised with the flock, not that much. and sometimes it does not matter.
Any breed suggestions for a mixed flock? I'm thinking Sussex, Ameraucana, Polish... or even a Silkie or a few bantams. Maybe.
What are your goals? Why do you want chickens? It's hard for me to make breed suggestions if I don't know why you want them. Are size, number, or color of the eggs important to you? Are they just pets? Do you want a rainbow of feather colors and patterns? Is it important to you if they do or do not go broody? What I want is different to what you want.
I know you are dreaming and I don't want to rain on your parade too much. You can try practically anything and maybe with a little help from us your odds may not be all that bad if you get a chance to live that dream.
We can always come up with reasons why you shouldn't do something because you might possibly have problems. Failure is an option but you can't succeed without trying. And sometimes their are solutions. Many feed stores sell a minimum of 6 chicks so find a neighbor willing to split an order. If you hatch eggs or buy chicks you are likely to get males, sometimes most or all will be males and sometimes most will be pullets.. You meed a plan to deal with them. There are many different strategies to deal with that. Since you free range, Silkies that can't fly or Polish that can't see might be more vulnerable to predators but many people free range Silkies or Polish and do fine. If you free range the others are vulnerable anyway.
Since you have not been through it I don't trust you when you say you have enough room. With integration it is not a matter of square feet per bird. The quality of that space is important. You may be OK with that, you may not. But don't let that stop you from dreaming. If you decide to go forward many of us can help you improve the quality of what room you do have or we may tell you to provide more room if needed. We can help you with the practical aspects of implementing your dreams. But don't let what might be a problem stop you from dreaming. Or asking.