Challenging Smite Damage reduction + ability damage reduction stacking?

So I was wondering, does the 20% damage reduction you get when you smite an enemy champion for 2.5 seconds stack it's effect with a champions ability to have similar damage reduction like 's Molten shield or 's Unbreakable Will?

If it does, is there a cap to the stack? Because I imagine that if had 95% damage reduction (20% Challenging smite + 75% ultimate) from everything but true damage for at least 2.5 seconds, that would be pretty strong. If it doesn't, is there a priority towards damage reduction from abilities versus the smite's effect, even when the damage reduction you get is weaker like a level one Molten shield?

I tried looking through the search results because I couldn't find an concrete answer to this question specifically, just about item damage reduction stacking multiplicative suck as and .