I asked ESO on twitter if you could mix armor types (such as wearing a medium armor cuirrass and a heavy helmet) and whether you could get some benefits of each of the armors if you did so. Answer:
Hopefully you get some of each benefit but not as much as if you were wearing a full set of the same armor type. So if I wear a light armor robe and medium boots/helmet/gloves then I should get some boost to magica and some boost to stamina but not as much of either as if I wore all medium or all light.