In answer to the question "Who can make use of a Coin to become a Denarian?" I'd answer nobody. Although it may seem like semantics, I think it's a fairly key point: it's not the bearer who "uses" the Coin; it's the Fallen in the Coin that uses the bearer. And before someone points out certain Denarians who have retained a semblance of free will by forming a partnership of sorts with their coin's Fallen, I'll point out that the Fallen can choose to turn off the power at any time, or select what power to grant the bearer on an instant-by-instant basis. The coin is not a 'magic item' to be wielded; it's a conduit by which the Fallen can tempt the bearer with offered power.
So what sort of creatures can a Coin make use of, then? I don't have a solid answer, but I'll substitute for facts with the following complete conjecture:
I think that the goals of the Fallen involve the corruption of God's design, which means mortals -- including Wizards, but not including most other monsters. I suspect that the Fallen could exert control over many such monsters, but generally speaking choose not to. Why? Well, perhaps they feed on ... well, on "the corruption of God's design". Monsters acting monsterously doesn't do it for them, it has to be a mortal who chooses to act monsterously.
Or, at least, that's one possible explanation.
