Question: Is it true that these bosses cannot be killed once you are above their level?
And is it also true that these bosses give codex entries upon defeating them?
You can outlevel them and come back. A friend and I have Duo'd The First and Grandfather at around level 30.
And yes, they give an Epic Enemies Codex Entry upon defeat.
A question of my own: Can Empire players find Subject Alpha, or is it in a Republic-only version of Taris?