Zandra

Mitigation stacks, although you cannot stack multiple potions/crystals with the same effect on top of each other. I believe you're allowed to use one active effect per item of different professions- for example, a mitigation crystal (alchemy) + some sort of HP boosting food (cooking). 
Don't quote me on the second though, because the old rules that clarified this seem to be missing, as they used to be listed under the old Professions guide (which has now been deleted).

Certain effects stack on each other, like the alchemist's damage enhancement and the cook's alcoholic enhancement.