I started with two white rocks and a dog house. Then i bought 8 bantys and they did just fine. Then 4 and so on now i have a 14 x 14 coop and 15 chickens, standard and bantys all mixed in with each other and 17 chicks. Most of them i bought at different times and added in the same day. Had no fighting when i added them.
So i would say you can i havent had any problems with it.
I tried putting new hens in with some that I already had and they did not get along at all. I waited a few days to see if they would stop fighting, but they never did. I ended up taking the newbies out and putting them in a seperate cage.
I have heard that you can put newbies in with the others after dark and they will wake up with them and get along fine, but I have not tried this yet.
It really depends on the birds. If you're not sure of the health of the new birds, you should wait to introduce them to the flock until you're sure they're healthy. We're about to add some 7 week olds to the flock. I have a separate cage/tractor that fits inside the coop for the new girls, so the other hens can see them and interract with them, but not get at them. That way within a couple of weeks I can start letting the new girls out on supervised visits a little bit at a time to make sure that no fights break out. They shouldn't, though. So far I've been really lucky adding new birds.
Sure you can add them together. They will fuss for a few days until they establish the peck order. Just remember to quarantine any new chickens you get to make sure they aren't diseased, or carrying parasites. I have also heard that adding them at night eases the adjustment, but I've never tried it. I imagine they still carry on some. Its just natural for them to establish that peck order, but I've never had any serious injuries from hens. Roosters, of course, can be a bit more aggresive with one another, but in a big enough flock they will work things out to as long as they aren't too gamey.
It depends on the size of the established flock. When I have an older flock of 30 or so birds and want to add 10 or so youngsters, the older flock picks on the younger chickens and can kill them. So I split the older flock, leave about 10 in the coop for a week or so with the younger birds. Then when I add the other 20 chickens back to the flock the older birds work on re-establishing the pecking order. By the time they are done with that, they don't notice the youngsters.