It is good to be thinking about these things ahead of time. Depending on the breeds you have, you may or may not have a hen go broody. I had been keeping them for years and even had some breeds said to brood often but my first broody was a BSL. Go figure.
You don't HAVE to separate the broody hen from the flock but it is a good idea. First, while she is brooding it will keep the others from squeezing in and adding their eggs to her clutch and second, after the chicks hatch, they will be safe from the rest of the flock. Separating them allows you to set up a feeder/waterer for the chicks and know that they will get enough to eat and drink, without the older hens eating it all.
Regarding your second question, yes, the broody hen will sit on anything that vaguely resembles an egg and won't discriminate between those she has been sitting on for two weeks and one that was just laid. For that reason, its not really a good idea to let the others keep adding to her nest. What will happen is that when hatch time comes, those set first will hatch and she will then abandon the nest so she can care for her chicks, leaving eggs behind that are further behind in development. So it is better to give her a clutch to sit on and don't add to it after she starts sitting.
