Its also possible they adapted or "helped" themselves like they "improved" the Jem'Hadar and the Vorta- maybe they're just renegade freaks who broke their society's rules on genetic manipulation and were oppressed or there were arrest attempts for what they did.
After all, as mentioned, they seem to remember being solids.
And for the other point- the Female Founder kindof hesitates in "the Search" as if "infant" isn't the right term, just one that Odo sticks on, so she decides to go with it. That flexibility is used in the DS9 relaunch. Odo wonders why he was "sent away"- a good point, if you're worried about being hunted and attacked- would you send your children away with no education or training?