Well, then that'd just make the change from Stone to Iron less jarring and more comfortable for the characters of Pohatu and the Po-Matoran. I don't know how brittle natural iron ore is, so I'll have to look into that. And, as I mentioned earlier, he could always keep a source of purified iron on hand, kind of like Toph's meteor bracelet or Katara's water pouch in ATLA.
Anyhow, those Toa Disks are in turn made FOR their respective Toa- Although who exactly creates them is up in the air, as far as I know, but I don't think the Toa Disks are necessarily more powerful than generic Kanoka, due to them being of mostly honorary status. I could be wrong, though.