On the Pokemon manga special volume one on the page that describes Red's Pokemon and the HP stat bulbasaurs typing doesn't say poison as type 2
Bulbasaur, Ivysaur and Venusaur have always been Grass/Poison.
Source: Knowledge and Experience
Source: Experience
Yes
Apparently in the original Gen 1 game every Pokemon only had 1 type. Dual types were introduced from Gen 2 (Gold/Silver) onwards which is when Bulbasaur became a Grass/Poison, Chsrizard became Fire/Flying etc