Does Seviper's "More Poison" Ability stack with the effect of Toxapex GX's "Super Intense Poison" attack?
Yes, you first put 10 damage counters for the Super Intense Poison attack and then you get to add 1 damage counter for each More Poison ability.
Source: TPCi Rules Team (2017-08-03)

Relevant cards

Note: All applicable cards may not be listed below.
SM2_136
SM2_154
SM2_57
SM3_50
SMA_SV15
SMA_SV15