I don't give the hen the eggs I want her to hatch until she's been broody at least 48 hours. To me, that means sleeping on the nest two nights in a row. Sometimes hens will ease into being broody, spending all day on the nest, growling, etc....but still roost at night. I'll leave bait eggs there during that time to encourage her, but until I'm sure she's tight on that nest night and day she doesn't get the real deal.
I have my broody hens hatch in the coop probably 85% of the time and have no issues. A large space is key here, the chicks have to have space to get away from the mature hens. Broody momma is usually hormonal enough no one messes with her babies. My only issues have been a very, very Omega hen brooding at the same time as an Alpha hen, when the chicks hatched the Alpha tried to kill them. Otherwise, the hens look at the babies and sometimes give them a peck or two, but no biggie. The babies run screaming back to momma, who basically says "I told you to stay out of her way" and everyone's good.
I've always had bantam hens raising large fowl chicks, never the other way around. But, I do have bantam littles out of the brooder in a pen with large hens with no problems. I don't see why you couldn't get bantam eggs for her. Myself, I'd go with an even dozen, cause I'm greedy . But if you place a dozen, say 8-10 hatch, half of those are males, that could easily knock you down to your three more pullets. Just prepare the kiddos from the start that all the birds won't be living happily ever after with you and you can sell off the extras after she weans them.
Incubating and brooding is so much easier with a broody hen! She does all the work, and is thrilled to do it. Plus, IMO nothing in the chicken world is better than watching a momma introduce her babies to the world. And seeing a chick riding on momma's back is priceless!