It is unkown if the Skill will activate with 100% certainty, but I have personally yet to see a single 200 Combo Level card that didn't activate a Skill or Combo.
So even if it's not exactly 100%, it's pretty safe to assume that they will.
P.S. Just try to attack Zoser, all his cards are Combo Level 200. When I did, he activated every single Combo and Skill that he could possibly get