I like the idea of having different resources appear in varying qualities, but it might just be a little to complex to be worth it. It could not be added immediately, since they have reached a temporary cap on the number of resources allowed. Therefore almost everything else in this post are probably bad ideas.
It would be far more realistic if there were buildings needed to refine a resource into a more usable form(a smelter could turn crude iron or copper into their pure form, which might be present on the map but much rarer), or to create some new resources out of multiple old ones . The Tailor (a building currently available only to Kuriotates, I think) could produce various types of clothing, a new luxury resource, instead of just improving the gold or happiness of existing resources. The new resources could then be traded. Perhaps multiple metal should make alloys which are treated as separate resources, or are just both needed to give the metal upgrade (copper isn't enough to make bronze, you need tin, or zinc if you prefer brass).
I would prefer giving the various resources slightly different bonuses, not just +1 strength per level (If you add enough levels, you make a civ without metal resources far too weak). Then, perhaps the metal upgrades could be given in the way that the navel upgrades longshoreman crew, buccaneer crew, and expanded hull are currently. They would be upgrades that you could freely switch between (assuming the unit is in a city with all the metals available) and which are not tied to the unit's xp. This might answer the problem of a metal like lead giving the movement penalty along with the heavy promotion.
In Ancient Roman times, bronze was associated with witchcraft, probably because the most arcane rituals were thought to have been passed down from the bronze age unchanged. Perhaps adepts should be able to get this one metal promotion, and it could offer a slight magic bonus?
I remember reading somewhere (in a parody of the King Arthur story which I truly disliked) that elves and other magical creatures hated the use of iron because it required delving into the earth, hurting mother nature, but that they were fine with Meteor Iron, which they used to fashion such things as Excalibur. That probably does not fit as well in FfH, but it might be interesting to not let elves use normal iron.
You do realize that stone and Marble are currently separate resources, right? I could see adding different types of stone, but again I would prefer different bonuses. For instance, some stones add more to a building' prestige but aren't physically as strong or durable. It would be gneiss if a castle built with granite could offer more defense but one made of marble had more culture. Perhaps different types of stone should also effect the strengths of certain golems.
There are lots of other resources that could be split up, such as horses(some are better for work and so give extra hammers, other are faster, giving extra movement, while still others have the strength needed to carry a knight in full armor), sheep (cashmere, anyone?), gems (diamonds, emeralds, rubies, turquoise, etc), and probably every other resource too. Of course as stated in the beginning, most of these would add extra complication to the game and probably would not be worth it.