Are Generation 1 Pokémon still able to hatch from eggs if they have a Generation 2 preevolution?
Solution 1:
Update: SilphRoad did the research and the answer is NO.
Original answer: I have not hatched any of the adults personally, but yes, adult versions of the Pokémon were reported to be hatched from eggs obtained after the baby Pokémon have been introduced (e.g. here, or here). So as long as we trust the report authors, the answer is yes.