Hello Jojorose,
I have done a combination of all of the above--I have slipped newly incubated chicks under a hen that I allowed to sit the full 21 days on wooden eggs, I have allowed my hens to hatch their own (and their sisters') fertilized eggs, and I have bought newly hatched chicks and slipped them under my broody hens. I have had success with all the methods. In addition, I have slipped newly incubated chicks and store bought chicks under hens who have 2-7 day chicks they incubated on their own. I have yet to lose a chick. I always slip the chicks under the hens at night, then observe in the morning when they start moving around.
At this very moment I have hens who have a combination of all three: their own brooded chicks, chicks from the feed store (you can be pretty sure you'll get hens), and chicks that I incubated and put under them at night in my chicken yard. The mama hens know no difference. They tend them all equally. I have almost 40 chickens, good-sized coops with plenty of space for all, and a decent sized chicken yard.
Recently two of my hens shared the same nesting box that had a divider that mostly separated them. They went broody at the same time. When the chicks hatched, they were small enough to freely move from one nesting box to the next. Now the two mama hens and all the chicks are an inseparable family. They are always together. Neither the mama hens nor the chicks seem to know who belongs to whom. It is lovely.
I wish you the same happy chicken families.
BTW, I also allow my hens to brood for the full 21 days before putting any chicks under them, assuming that they are programmed to sit for a certain time. I want it all to feel as natural as possible.
Best of luck!
MaryZoe