ELB, am I correct in thinking you are asking for help with the survivor skill "poison weapon", and not the warrior skill "poison damage"?
If so, the skill description implies that you can choose different poison strengths:
"Putting poison on a weapon yields X amount of poison charges. Strength of poison depends on the type of poison used. Poison effects are not accumulative"
Or at least it implies that different strengths can be used. It may very well default to the strongest poison potion you have.
