A lot of that depends on how you feed them and your plans for the cockerels. If you separate them you can feed them differently if you want to. If you are raising the cockerels for meat you might want a pretty high protein feed to enhance growth.
A normal way to feed pullets that will become a laying flock is to feed them a relatively high protein Starter to get them feathered out and off to a good start, then switch to a bit lower protein feed to kind of get their body growth, skeleton growth, and internal organ maturity all in sync. Say a 20% Starter and a 16% Grower. Of course many people don’t do this and feed a fairly high protein feed, say 20% protein, throughout their growth. A 20% feed called “Flock Raiser” is available here but might be called something else somewhere else. It is often used to feed a flock where some will be layers and some will be for meat. There is no one right way or wrong way to do this. We all do it differently and most are pretty much successful.
There won’t be a lot of harassing behavior of the cockerels toward the pullets until the cockerels hit puberty. When will that be? I don’t know, could start around 3 months, maybe not until 4 months. I personally don’t see a lot of benefit in separating them until that starts to happen but you can separate them as soon as you can clearly identify them if you want. It won’t hurt anything.
You can pick one of the cockerels to stay with your pullets now if you wish or separate them all and wait until they grow out to pick which one you want to keep. From reading between the lines your post separating them all might work best for you but I go different route.
Once you separate them, separate them. Like Sour said, back and forth is not good.
You can try to separate them as you identify them. I’m kind of surprised you haven’t seen some puberty behavior by 16 weeks but it should not be long coming. It’s possible it won’t be all that bad since each chicken has its own personality and each flock has its own dynamics, but it usually does get pretty noticeable. The cockerels fighting among themselves is not that big a problem for the pullets, it’s when they start to force mate that the pullets get excited.