What abilities do or do not work together? Mainly, I want to know if having 4 Helltooth sets and the exact same skills on each is valid. HT 2 pc puts Necrosis on mobs, doing 1500% wrap damage over x seconds. Does this stack between 4 toons, each independantly doing their own damage? Does 12 Firewalls (3 from each with Jeram's Bracers) stack? Does the cold ground effect from Acid Spit, frozen ground rune cause 4x damage or just 1x damage?

This is also just a general "what is the recipe for stacking, or does everything stack with everything?" type question.

Thanks!