Anyone done testing to see how Rift Guard stacks with those Greater sigil slot fillers that give you a "15% mitigation for 300" or whatever amount of damage?

Those 20% reflect drops from rifts for example don't stack. I don't want to burn money/time testing if someone has already done the footwork and is willing to share :x

And if they stack, how? For example does the (lets pretend base rift guard with no improvement) 15% rift guard add to the 15% from the sigil proc when damage calculations are done and you end up taking 30% less until the damage from the sigil proc is exhausted?

If these are not viable, what greaters do you use? Also, is it possible to put in two of those Greaters that buff endurance by 55 when struck (to increase the proc rate and potentially keep it up most/all the time in combat)?