Poison is only weak to Psychic and Ground, though...
Star Dood wrote:
Then there is Rock and Steel, which are both crushed by Grass type moves.
I'm sorry, what? Steel RESISTS Grass. Very well.
I'd try to have something dedicated to just anti-Steel, because their complete immunity to Poison could ruin your day very quickly, and you really don't have anything to beat it (Tentracruel's Water attacks would deal normal damage, but then again... ehhhhhhhh don't rely on it). I'm pretty sure Golbat can learn Fire Fang or something, which would be great since a lot of big Steelies like to pack Earthquake (and Golbat can just fly right over it).
To be honest, I'd try to have as many dual-type Pokemon on the team as you can (without repeating type combos of course). While Poison moves in general are really nasty and can leave your opponent sore for a long time (especially in Gen 5... hot damn Acid Spray is amazing) they also have some pretty bad coverage (a few types resist Poison easily and it's only super effective versus Grass). Unless you have a versatile Poison type that gets a wide range in their moveset (like Golbat) then personally I wouldn't want them around unless you need a space filler.
... While I'm at it, give Venusaur EQ. You could use the coverage against Electric.