Am I able to play a 1 HP unit with We'z Bigga in a Zone with Scrap Heap?

Due to the Greatswords ruling I would say yes. You play a unit first and then look for effects. Scrap Heap is a constant and I am active player so I would choose to rule Scrap Heap before the killing rule. Or how is the killing rule to time?

jogo said:

You get no say about the orders the rules apply, only conflicting card effects and there really is no conflict here. The card cannot leave play unless it has entered play so We'z Bigga does not send the unit from your hand into your discard pile. It must hit the table but it does so with one damage, this means we check to see how many HP it has in comparison to damage tokens. What we see is that all Orc units in that zone gain +1 HP and the unit is an Orc unit so its total HP is 2, which means the damage tokens does not equal or exceed its current number of HP.

The thing to remember is that a Constant Effect is a fundamental change in the game play state so anything that requires us to check the rules or condition of a card must take all Constant Effects into account before we determine how something is resolved.