Is there any difference in resistance between the 3 mil skeleton and the 6 mil robust one? Or is the robust one just named that because it has 3 mil more health?
Just more health. They both 18.2k spell and physical resists.
Yup. The 6 and the 3 mil skeles take the exact same damage values as one another; just different size health pools. I believe it's the same will all the bigger Dwarven guys too, but I've not spent much time with them, tbh.
The only exception I'm aware of is The Precursor, which has lower health and resistances. You can easily see this by light attacking any regular target dummy, then doing the same light attack on The Precursor. Pretty sure Precursor has the same resists as Overland mobs and the like.
You can turn on damage numbers temporarily to see the effects of this stuff, if you normally play without them. Give it a try.