I believe that a coinshot could push on pure sodium, but could one steelpush salt? Can steel and iron allomancy effect compounds with metals in them, or just pure metals?
Inquisitors can see trace amounts of metal in people. They could be seeing the ionic compounds, or maybe they are only seeing the tiny amounts of pure metals.
Allomancy works because the shapes of the molecules in the metals act as a gateway to the power of Preservation, similarly to how the Aons from Elantris can access the Dor. We have word that the planet chooses how the magic system manifests, but I think (not sure on this one) that Brandon has also said that since ascending, Sazed could change which metals powered which powers. Because of this, I don't know if Scadrial or Leras originally chose the sixteen allomantic metals.
As far as we know, steelpushing and ironpulling can effect any kind of metal except aluminum and some of its alloys. If you can't push or pull on an alloy of aluminum, this means that the force isn't acting directly on the metal atoms. For example, if duralumin is allomantically inert, it shows that the force must act on the compound, or else you would still be able to push on the 4% copper in the alloy.
My theory is that an allomancer can only push or pull on pure metals and alloys. The compounds that can be pushed or pulled were chosen by Scadrial or Leras. An allomancer cannot push or pull on an ionic compound because even thought they have metals in them, they are not metals.