Mechanics questions

[Wandering Spirit]

Hi! I was thinking about building a team around defensive buffs, noticed that the engineer has a couple, that got me wondering... 

Do critical hits multiply special damage types: soldier's plasma, sniper's poison, hydra's fire? 

How do -mp and +mp stack? If I have -mp gear, does the game remember my "base" mp for purposes of calculating heat reduction, if I have a +mp buff but I'm sitting still? I mean, can I get extra heat reduction with a -1mp armour and +3mp rallying charge if I don't move or do I have to have an active max MP of 5 or under to benefit? 

[fallen]

-MP and +MP stack, yes. Any source of -MP reduces your base for heat reduction calculation. However, only the final, summed value counts (-1 + 3 = 2, so no bonus to heat reduction, just move faster).

Critical hits do not multiply special damage.

[Cory Trese]

You are clearly thinking along the right lines if you are asking this question. It works the way you'd expect it to work, in balance with Heat and letting you trade Debuff into Buff for a mixed result.