Armor and runes

I understand all armor needs to be the same level for all armor pieces. If you upgrade 1 piece and not the others the damage taken to the lower pieces is 100% according to wiki. My question is when runes are applied do they need to be the same on all armor. For example if I add +10 armor to chest piece it is now 10 higher than all other pieces. Does this reduce the effectiveness of the other armors? What is the effect on other pieces? I guess what this is getting at is can the runes be different on the pieces of armor? some + armor some + health etc?

Whenever you are hit, you are hit on one armor piece. Different armor pieces have different chances of being hit. Chest is 3/8, Legs are 2/8, Head, Arms and Feet are each 1/8.

Insignia usually offer armor bonuses. These apply only to the armor piece they are on. For example, if you have 60 armor and you put a +10 armor insignia on your chest, you will have a 3/8 chance of getting hit on your chest and having 70 armor and you have 5/8 chance of not getting hit on the chest and having 60 armor.

Runes are different from insignia, they offer other non-stacking bonuses (only runes of vitae and attunement stack). These don't really matter on which armor piece they are put. The only thing to take note of when runing is what head piece you have.

What? If you upgrade one piece (let's say the chest), you'll take less damage then you used to when you get hit in the chest and the same as you were everywhere else. The armor level on each piece has no effect whatsoever on the other parts. Wherever you got that from is nonsense (unless you interpreted it incorrectly).

No. There generally isn't a point mixing them though. The only exceptions are when one is global (non-stacking, affecting everywhere) such as Bloodstained (for Necros) and Stonefist (for Warriors).

As above. Armor level on each piece has no effect whatsoever on the other pieces.

Yeah, if you really wanted to. Generally you'll want all +armor or all +health; unless you're looking for some specific amount of health mixing insignias is usually just annoying.