Iron was an Elemental Power. This element was used by Toa as well as Turaga of Iron and Fe-Matoran. Toa of Iron had almost perfect control of the element. The element gave Matoran extreme endurance. Turaga abilities were somewhere in the middle. Skakdi of Iron only had access to the element when they used their powers with another Skakdi.
It has been confirmed by Greg Farshtey that beings other than Toa couldn't combine powers of any sort to create a Protodermis cage. It has also been confirmed that as long as each of them wielded a different Element, any six Toa could create a Protodermis cage.