1. will she take to a couple of chicks if i place them under her after a couple weeks of sitting on eggs? or should i get fertile eggs?
Most will take to very young chicks placed under them. It is not a sure thing but it usually works. Cochin and Silkies are both broody breeds, so I think your chances are really good. I have had luck putting the chicks under the broody at night with it completely dark, using as little light and commotion as possible. The danger time is when the hen wakes up in the morning. The younger the chicks the better chance of it working.
Fertile eggs are also an option. There are risks associated with both methods but either should work. If you do decide on fertile eggs, you might want to read this link. Whether you decide to separate here or not while brooding, there is good information. If you do decide to not isolate her, please pay attention to the marking eggs and checking under her every day comments.
Isolate a Broody? Thread
https://www.backyardchickens.com/forum/viewtopic.php?id=213218
2. to what extent is it necessary to separate her from the rest of the flock; more info about flock below incase that sways answer.
People have been successfully doing this both ways for a long time. There are risks and advantages both ways. I prefer to have the broody raise them with the flock so Mama handles the integration issues. They still have to handle their own pecking order issues when they are weaned and those can sometimes be a bit rough, but the integration issues are the most dangerous. My normal method is to separate the hen and chicks for a couple of days in their own area so the chicks can learn to eat and drink without interference from the older hens, but then put them with the flock.
Which works best for you will depend on a few things. Partly, how good a broody is your hen? Most broodies are very protective of their chicks. If another chicken threatens her babies, Mama usually goes ballistic. She has such a bad attitude that the other chickens usually very quickly learn to leave her chicks alone when they are around Mama. Now if they wander off on their own or get where Mama cannot protect them, then you have another issue. I remember a specific poster on here that locked them up in a separate pen. The chicks could get through the fence but Mama could not. That was not good when the chicks made their way out to the rest of the flock. Mama could do absolutely nothing to protect her babies. Sometimes you get a broody that does not protect her babies, but that is fairly rare.
A second consideration is the personalities of your other chickens. Some chickens are brutes. Some individual chickens will go out of their way to kill any chick they can. In my experience the other hens are the worst, but it is also possible that a rooster could be a threat. My flock is pretty laid back. I've had 2 week old chicks eating out of a feeder next to grown hens, with Mama several feet away. Sometimes the other hens let the chick eat next to them, but often another hen gives the chick a pretty good peck to remind it that in chicken etiquette it is bad manners for a chick to eat with its elders. The chick then runs at top speed back to Mama. Mama, in this case, comforts the chick but does nothing to the other hen. I guess it takes a flock to properly raise a chick and teach it manners.
That peck was a pecking order type peck. Hard but not meant to kill. There is another type if pecking which is a flat out intent to kill. This is where Mama usually quickly steps in. It depends on the individual chickens. I cannot tell you what the personality of your chickens is.
The other thing is room. If they free range, Mama has plenty of room to work with. Your pen is kind of small. If Mama has enough room to work with, she can usually protect her babies. The more room she has, the better job she can do.
I can't tell you what the right way is for you; just try to mention some of the possible problems I see.
3. if she is raising them and it stays above freezing (its been about 40 at night/50 day temp here) do i need to suppliment with a heat lamp still?
I never would. Mama's heater does not go out with a power outage. If they get cold they will go to her for warmth. You will probably be surprised at how often and for how long they enjoy being out in the cooler weather.
Good luck!
