I think Poison should hit Fighting for SE mainly because of the movie cliche of most Martial Arts Masters getting Poisoned when they can't be beaten outright, and yah Fairy for the same sort of reason.
Poison could also benefit from a couple more moves:
One that can hit Steel (May lower Defense?). One with a chance to Paralyze (Venom?). One with a chance to Burn (Corrosion?). And a Poison weather like Acid rain that can stack on top of other weathers. It could have the following effects: Doubles Chance to Poison. Poison Types heal 10% Hp in Acid Rain. Non Poison Types take 10% Damage in Acid Rain. Steel types lose a stage of each defense upon entering Acid Rain or take damage. Either way, Steel should corrode in Acid Rain, maybe it can lose it's Poison resist. The idea of the Acid Rain weather should be to make having the Poison type be a gift, and not just having Poison moves :p.
Taylor Lautner - Giving me strength since the New Moon