It certainly seems like Mistings of all the metals are possible, but the problem is knowing who is who.
All the "basic" metals -- the ones Mistings are known to use -- are present in trace amounts when one drinks from a metal cup, takes liquid from a metal pipe, eats from a plate, etc. Thus, Mistings typically discover their powers "accidentally" when they ingest the metal.
That's the only way to know if you're a Misting: by burning something.
This is as good a reason as any as to why there are no Atium, Malatium, Gold, or Electrum Mistings; all of those metals are expensive and are not a part of a normal person's diet. The only way these would get into your reserves would be if you ate them, which is not something a "normal" person would do. Mistborn are the only ones who use those four metals, because they know they can use them.
For all "we" know, there could also (technically) be Mistings that are able to use Aluminum and Duralumin, but they'd be completely useless, since that pair neutralizes or augments the other metals. Aluminum and Duralumin would drain the Misting's reserves of other metals, but the Misting would not use the other metals anyway, so what's the point?
By the way, it occurs to me that there ought to be an eighth pair -- one that neutralizes someone else's metals, and another that augments someone else's metals. Perhaps that will be revealed in the third book. Or perhaps it won't.