Does Cassiopeia Work with Teemo?

Yes, the damage from the poisons stack, and for Cassiopeia's abilities any poison counts for the additional effect. All damage spells stack.

Does Cassiopeia E work with teemo?

Twitch, Cassiopeia, and Singed will now work good with Teemo. With Teemo's new passive, giving him bonus damage on-hit to targets with ongoing poison debuff.

Does Cassiopeia W poison?

Active: Cassiopeia spews venom in an arc, leaving toxic clouds for 5 seconds. Enemies in the clouds are continually afflicted with Debilitating Poison, inflicting a decaying slow and grounding them, prohibiting the use of Movement abilities. They also take magic damage per second.