For the merc, from my experience, the lower level you hire them (level 18 for an act 2 merc) the better stats they will gain over all. From what I remember, I've hired mercs at 19 only to find they couldn't equip X weapon, but hired at merc level 15, then leveled to 19, they were able to equip X weapon. I would only imagine the same would take place with their resistances. I've yet to try it out with my defensive merc I hired in normal. I suppose I could rehire one and see if the stats were equal.