We also know that they couldn't get pregnant before Helo came along. They were identical in all other respects. They didn't know why. But apparently males could impregnate human females, but none apparently ever tried.
Oddly enough, Tigh managed to get Caprica pregnant, but not have kids with his first Cylon wife, who was an older Six. (At least, that's how she appears in Tigh's hallucinations.)
I also remember that the secret ingredient in Athena's case was Helo's love for her.... which I don't think applied to Chief, whose wife might've loved him, but I don't think it was a mutual thing.
They couldn’t get pregnant Cylon to Cylon, but they figured out they could Cylon to human with the “missing ingredient”.
Ellen wasn’t a Cylon, that’s just Tigh’s weird hallucination to self justify his behavior. Tigh hasn’t been particularly sane since the beginning of season 3. Lost his eye, killed his wife, found out he’s a Cylon, just not good for his brain.
Keep in mind the Final Five are different than the rest. All bets are off when it comes to who and what they can impregnate and why.
“We also know that they couldn’t get pregnant before Helo came along.”
We don’t know that yet. I think the cylons have been producing hybrid offspring all along only we don’t know it yet.