That was my point before: people wouldn't. Nobody would waste any metals on someone they didn't know was capable of Allomancy. The only reason anyone knows about any Mistings is because the eight "basic" Metals are found in trace elements in things like silverware and metallic goblets. Nobody force-feeds other people Pewter to see if they're potential Thugs; they just notice an increased strength one day and figure out that it's because they ate tomatoes or something from a dish that was made of pewter. Similarly, Tineyes just experience greater sensitivity after drinking from a tin cup, etc.
Of course the metals they "discover" their powers with are impure, so they would obviously get a greatly diminished effect (just like Vin's "luck" was incredibly limited compared to what she can do now!) However, the idea is that pretty much all of the "basic" Metals can be ingested unintentionally, and pretty much everyone has at least a few of them in their stomachs at any given time.
Mistborn, since they are able to burn any metal, know they can get away with burning stuff like atium, gold, electrum, duralumin, etc. However, common people wouldn't even dream of wasting such precious metals unless they were sure it would work, or they didn't mind gagging up a few rare metals if it didn't. Atium Mistings, if they even exist, would be tremendously rare because Atium itself is worth a fortune, and nobody wants to swallow any in case they can't use it. Sure, they could probably puke most of it up again (like Vin did with the lead ball) but if it's only a slim chance, normal folks probably wouldn't waste their time.
Heck, for all we know all Nobles have some sort of Allomantic power; it's just that the people who seem "normal" either have yet to Snap or only have the ability to work with one of the more exotic metals -- and would think it a waste to try them all and see.
It's even possible that there are Aluminum and Duralumin Mistings, although these would be completely useless and about the same as normal people -- albeit normal people that can break down metals in their stomachs faster than most people.
And the more I think about Aluminum and Duralumin, the more I think there ought to be an eighth pair: one metal that augments someone else's power (like an External Duralumin) and another that inhibits someone else's power (like an External Aluminum). Granted, the External-Aluminum family would be like cheating, in a way, but given the pattern, this really would make a lot of sense.
So far, what we know is that there are two metals for each type of effect: one for the Internal side of the effect, and one for the External side of the effect. The alloy and the base metal form a pair of either the Internal or External attribute of the effect, with opposite results. For example, Tin makes the user (Internal) physically sensitive, while Pewter (the alloy) makes the user (Internal) physically more durable and powerful. Together with Iron and Steel, they form the "Physical" effect family; Iron and Steel are external effects.
Similarly, Atium tells what someone else will do in the future (External, Temporal, Future = Pushing?) while Malatium tells what someone has done in the past (External, Temporal, Past = Pulling?). Gold tells what you looked like in the past, and I would assume that Electrum tells what you will do in the future (another way around Atium? This would have a similar effect to what Vin did when she observed Zane observing her future actions, correct?).
Therefore, if the Effect for the Aluminum family is "Internal, Power" there must also be another base/alloy metal group that has yet to be discovered, which will either increase or decrease another person's power. It also remains to be seen exactly to what degree the effect occurs, and how quickly or slowly the metal burns. Malatium, for example, tells you someone else's distant past, while Atium only tells you someone else's very near future. If there is a similar effect imbalance inherent in the "Power" families, then perhaps the "External, Power, Inhibition" metal only decreases the degree to which another person can use metals -- rather than snuffing them all out the way Aluminum does. On the other hand, it's possible that the "External, Power, Augmentation" metal increases someone else's power exponentially, and for an extended period of time, producing an effect much greater than Duralumin would! There's no way of knowing.
Besides, metals can also have side-effects not necessarily reflected in their alloys or opposing families; Atium speeds up your reaction time so you can make sense of the future-images in time to act on them. Malatium, however, does not seem to have an observable effect on the user's thinking processes. In theory, Atium shouldn't even be able to speed up the user's thought processes, as it is generally considered an External effect.
There are exceptions to every rule, I suppose.
