No. Poison-types are immune to poison and toxic ailments.Can you poison a poison type pokemon?
If you counter under a substitute does it send back the damage done to the sub? If so, is it 25% or is it the total damage the sub took?
No. Damage to substitutes cannot be Countered, Mirror Coated, or Metal Bursted. Damage done to a Substitute doesn't count toward damage taken during Bide, either.