LOL Acid Rain. Sorry, the reason I laugh is because a weather condition with that name already exists in all the fourth gen games. It's a glitch that happens when weather is in effect and you KO an opponent's Pokemon with Pursuit. Here, check it out:
http://bulbapedia.bulbagarden.net/wiki/Acid_rain
This idea sounds kinda cool, although it's a bit complicated...I can't really see a seven year old understanding the concept of "lightning strikes releasing sulfur dioxide". Poison does need a boost, but I'd rather see it get a super-effective hit on Bug and/or Water.
It's not all about weaknesses and resistances, though. To be honest, what the Poison type really needs is some bulkier, stronger sweepers. Gengar and Roserade have great Special Attack, but even if Sludge Bomb got some kind of boost (like hitting an additional type for SE) they'd never use it, since it's basically an invitation to Scizor to switch in and Bullet Punch them in the nads. On the physical side, Poison's hopeless. Pokemon like Drapion and Toxicroak just don't have enough raw power to reach top tier.
Minor nitpick...Sandstorm boosts the
Special Defense of rock types, not their Defense. Also, a boost like this would break Walrein. He's already annoying enough!
I always thought Hail should make Ice moves more powerful, similar to Rain Dance with Water and Sunny Day with Fire. But maybe that would be too much as well.