Could be anything, could be that she's in the middle of the pecking order of her current flock and sees an opportunity to rise up in rank. Could be a ton of things, I wouldn't worry about it too much unless it gets bad. But if it does, separate the littles again for a while.
Yep, completely normal. The younger chicks aren't even on the hens' radar because they aren't laying yet. Once they start laying, the 2 flocks will incorporate, but don't be surprised if they're always in 2 separate clicks.
It's good that you have more young ones, they will outnumber the older...