Caprica Six's "Head Baltar" had shown up several times before! Not nearly as often as "Head Six", but he had a reveal sometime in Season Two, I believe, and he had an appearance near the start of this season as well. We never saw the two "Head" characters together like this before, though.

It had been implied recently that the skin-jobs don't age, but it's ultimately left unclear, and I think deliberately so. I approve of this.

My interpretation of the posthumous Starbuck is that she originated from wherever the "Head" beings are from, but is not allowed to be fully cognizant of this.