I have a flock of mixed large fowl and bantams. When adding birds to my flock, I have found the best way to do it is very slowly.
Are the bantams full grown or chicks? If they are chicks, wait until they are old enough to go outside, and fully feathered. I have a completely fenced off area in my run I put new birds in. They can be with the flock and see each other, but not touch each other. I leave them in there until they are like sizes. In your case I would wait until the bantams aren't peeping anymore before letting them be together. I would let them "mingle" supervised, for a bit, then put them back in the separate pen. It usually takes about a week of mingling, before I let mine be with the flock permanently. At that point, you can put the bantams with your hens at night, that way when they wake up in the morning, they will be with the flock. It seems to cause a lot less problems that way.
If you are buying birds I highly recommend quarintining them. I know of people who have lost their entire flock to new diseases brought on by new birds. By quarantine, I mean the new ones won't even be breathing the same air as your current flock. Use this time to watch the birds for any sign of illness. Many illnesses don't show up right away. A lot of people recommend a 30 day quarantine period. After that period, you can introduce them the same as young pullets. Have a separate part of your run, or a fenced area to put them in, and let them see but no touch for about a week or so. You can let them mingle a bit too. There will be chasing and pecking, but it shouldn't be too bad if they already have a chance to see each other before hand. They will sort out the pecking order, which may take a few days. But you can add the bantams to the flock permanently at night as well once you feel they will be comfortable in the flock.
I have never really had problems using this system and all of my bantams and large fowl live together very harmoniously.

hope this helps you and good luck.