1) Venipede/Scorupi (based on her guitar)
2) Ekans/Arbok (depending on what level she is.)
At least one should be pure poison, so that all 3 starters are neutral to it.
Poison Resists Grass attacks and Grass Types are weak to Poison Attacks. It won't be purely neutral. If you evolve Tepig into Pignite, Poison then resists your Fighting attacks as well.
With Poison Gym having two possible Pokemon in it, its most likely the first or Second Gym.