You seem to be right about the adoption as a political tool use by the imperial family (for example Claudius had his daughter adopted out of the family so she could be married to Nero) but it's seem that adoption of females began at the time of the repulic.
"And so died Tiberius, in the seventy eighth year of his age. Nero was his father, and he was on both sides descended from the Claudian house, though his mother passed by adoption, first into the Livian, then into the Julian family." (Tacitus, Annals, 6.51)
Obiviously Livia first adoption happend in the time of the republic.