There are a few considerations here, the primary concern being that unless metal in your setting is a heavily-regulated commodity with a strict government, metal will be very plentiful.
Another consideration is whether allomancy is very prevalent. If it is, then your answer to the previous question should be influenced by this. The more prevalent allomancy is, the more work will be done to keep metal supply regulated.
I had another one but I forgot.
As for implementation, I would make an Allomancy (steel) skill for "pushing," Allomancy (iron) for "pulling," and so on... This can allow for a range of target DCs for slow burning or "flaring." Any DCs I would provide would be arbitrary, but if you want to use skills as a stand in, I would recommend working them out for how you want them used. (Obviously, easier DCs will make for more frequent usage.)
Using metals as what will basically amount to spell components, refilling them on a regular basis might be a solution? This really depends on the setting.
Hope that my thinking was of some use to you! Good luck and happy gaming!
At the moment, I'm just considering iron and steel for a sort of iconic 'bad guy' thing and thinking about using telekinesis as an at-will, or nearly at-will (maybe a specific power point reserve that could be recharged 'easily') power, along with blindsight.
I hadn't wanted to consider it as a general purpose thing, because then it would be worth it to consider a different ruleset. More a one-off for a very memorable encounter.