Just as a bit of clarification I wanted to be sure that you understood the Roman elites' implementation of adoption is rather different than the commonly practiced form today.

Roman adoptions would often be political and could help form a symbolic link between powerful clans or families. When you look at imperial succession of the early empire we can see elder men being essentially named heirs of the throne by sitting emperors who had them formally adopted. This is meant to be seen in contrast to the form of adoption where parents raise a child from a much younger age.

I'll leave the rest of this discussion to someone more qualified. Apologies if you already knew this.

I am curious which families you are specifically referring to? I am almost through Goldsworthy's three books on the Caesars, and can't actually name an aristocratic family without blood offspring.