I never used a demon, but I would assume they do regular melee damage. It would be strange to design a melee fighter that did magical damage, especially since the undead summon uses a magical attack. I don't think summons or your creature get a bonus from your stats. That would penalize people trying to get support where their character is weakest, which is the main benefit of using a summon or creature. Playing a strength based warrior, I certainly found my creature did much better in the fjords (trying to take out a couple level 21 imps at level 17) with a ranger head than it did with a warrior head, and a little better later when I switched it to a mage head.
