I have no idea why certain characters cannot wear certain items. I get a generic message 'this does not fit unit' many times when I would think something should be usable, like boots on a human. I would suggest adding text to the item descriptions that would indicate who is eligible to wear certain items and/or an indicator when purchasing an item that shows that the person buying it cannot wear it.
Also, not sure if it's by design but I was able to equip more than two magic rings. Most games limit rings to one per hand, I have no problem with being able to wear one per finger but I thought I would mention this in cas it was a bug. In my case it was the "+5 hit point" rings, I was able to equip three.
