I believe all stat modifiers stack since they aren't temporary buffs like Horn of Winter.
As for Scourge Strike, it's your Big Gunâ„¢ ability. I'm assuming you have the talent Reaping, so you should start out with applying diseases, using Blood Strike twice (or Pestilence and Blood Boil with multiple targets) to convert Blood Runes into Death Runes. Now you have two Frost, Unholy and Death Runes, of which the Death Runes and one pair of Frost and Unholy are on cooldown. You can use the remaining pair of Unholy and Frost to cast a Scourge Strike (get the glyph, by the way, it'll save you from having to reapply diseases sometimes). The killer, though, is that the next rotation will allow you to cast three Scourge Strikes in a row, because both Blood Runes are Death Runes now.
Cue voice-over from Unreal Tournament: M-M-M-MONSTER KILL!
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Please "talk up" if your comprehension white-shifts. I will use simple-happy language-words to help you understand.