All right, so maybe I lied about there only being three magic systems in this book. It comes down to how you term the powers of Preservation and Ruin, who kind of blanket the entire system. There are a lot of things going on here, and—well, the truth is I don't want to mention all of them, for fear of spoiling future books. However, I'll give you a few rules to apply.
First, to these forces, energy and mass are the same thing. So, their power can take physical shape—as Preservation's did in the bead of metal Elend ate. Second, there is a bit of Preservation inside of all the people—and it's this that allows the people to perform Allomancy. It needs to be awakened and stirred to be of use, but when it is, a proper metal can draw forth more of Preservation's power. It's like the metal attunes the bit within the person, allowing it to act as a catalyst to grab more power.
Allomancy is not fueled by metal; it is fueled by Preservation. The metal is the means by which a person can access that fuel, however. If there were another way to access it, then the metal wouldn't be needed.
Preservation's touch on people differs. Some have more, some have less. This doesn't make them better or worse people—indeed, some most touched by Preservation have been among the worst people in the world. As Ruin later points out, there is a difference between being evil and being destructive.
Regardless, if a person can get more Preservation into them, they become better Allomancers. Hence Elend becoming a Mistborn. Like all people, he had the potential within him—it was just too small of a potential to be awakened through normal means. That little jolt of Preservation's body, however, expanded and awakened his Allomancy.
As a tidbit, that was a side effect of what that bead of metal did. It wasn't the main purpose of the bead, and if another Allomancer were to burn it, it would do something else.