For what it is worth:
I have this dilemma from year to year. I have never had a problem with adult hens killing or even bothering chicks-- all the hens completely ignore the chicks (at least mine do)-- I understand all flocks are different, this is just my experience-- instead, it is the mother hen fighting the other hens that is usually the problem (mother hens will do this even though no other hen is showing ANY interest in her chicks whatsoever--) the chicks are real fast too-- they stay out of the way.
Each year, I let a couple of hens hatch a clutch and raise them. One time (last year), I let a hen brood them in the main run with all the other adult birds from Day 1. It matters the "rank" of the hen. It matters the breeds you keep. My other hens were a bunch of Buckeye hens and 1 Buff Orp; they are pretty mild & never have I observed them in a "killer mode" (I realize this could all change one day). This particular mother hen, though my ONLY bantam, was an OEG and kept all the hens away from her chicks, but she wasn't overly aggressive. The hens all learned just to keep a short distance from the chicks & peace was maintained. Also, I had lots of room in my run. It was only when mother hen weaned them that the young chicks got run around by the other hens.
Also, in the past, I have had brood pens within the main run where the chicks could squeeze out but mother hen could not follow them-- the chicks ran out with ALL the other adult birds continuously & would run back when the mother hen called them or to get warm. The other hens paid the chicks no mind.
This year, as in the past, I had things split up so I currently have two hens (1 Buckeye & the old OEG Bantam) with chicks, 7 & 5, respectively & they are in brood pens separate and the chicks can't get out. I tried letting the OEG out with her 3 week olds but the Buckeye hens & the little OEG fought (and I was afraid they'd crush her and even though I know she would have fought to the death, I wasn't going to allow it to happen) so back in the brood pen they went. All I can tell you is: if you decide to do it earlier than the 4 months suggested (or you have to for other reasons), make sure there is plenty of room for mother & chicks to escape & get to themselves AND make sure you are out there for a couple of hours watching things-- this is what I have always done in the past.
Just as a side note: I have always been amazed at how particular we are at keeping our chicks in heated brooders (& my favorite, no drafts), but I have had mother hens out in the worst elements, cold (30s) & very windy (an icy wind), storming & there are newly hatched chicks running with their mother like little bugs & doin fine. I've watched them, worried, many times-- but I find the mother hen seems to "know" that they need covering and warmed up, takes out of the storm, etc.
