Hi!
Maybe you should consider the thing that each type of armor has special abilities beside the simple idea of protection. You told about the issue that a plate mail gives you a penalty on dexterity, this sounds realistic. Think about designing armor for special purpose. For example a hide or fur armour could give you a bonus on frost resistance so this would be your best choice when fighting in snowy areas or against a frost dragon, prefered by vikings or barbarians from the north. A plate mail could give resistance to electric magic but prevent you from swimming, we see the witch hunters on the plains with that. Leather gives you a bonus when sneaking while metal armour makes some noise. So the player might have different armours for different situations or even better choose different companions for a given quest. I think companions should be unique, in the first way it sound great when every character can wear every armour but it makes them ordinary. I think a knight should look like a knight and a wizard like a wizard and not vice versa. And a wizard in plate mail is just a paladin isn't he?