It transpires that "human-passing demons" are only half the story. Some demon species, like sand demons, look human, or almost-human, or Star-Trek-alien, in their true forms, but there are also several species of shapeshifters. Most of the second category couldn't be mistaken for humans in their natural form, but can assume human shapes with effort ranging from "slight constant expenditure of energy and/or focus" all the way to "ritualistically consume one human brain and one human heart from humans you recently killed."
Also, "species" is not always a particularly apt term when it comes to demons. There's plenty of precedent for different types of demon being mutually interfertile, for one thing. Sometimes you'll get a single clan or family of demons whose members all have common characteristics that don't correspond to any recorded species. And then you have the types whose population is maintained mostly by abiogenesis; or who are created intentionally or unintentionally by some other type of demon; or who are unique, or single-digit in population, and haven't been recorded reproducing at all.
This and more, Kaylee and Tag and Nausicaa will learn in their researches over the next couple weeks, in between studying vampires. (It's easier to wound a vampire with any wooden weapon than it is a human, not just a stake - and much harder to harm them with metal or other traditional weapon substrates. It's hard to tell exactly how pierced a vampire heart needs to be by a stake, since successful stakings don't leave corpses, but the consensus is "probably needs to pierce at least one ventricle to a diameter of more than half an inch". Holy water is an effective ranged deterrent but you're not going to kill a vampire with it unless you trick them into taking a hearty swig of it. Crosses aren't any more physically harmful than holy water but they're strongly psychologically aversive. In general the people who can invite a vampire into a house are its legal residents, but there are plenty of weird edge cases, here are an example dozen, can you guess what would happen in each case, answers printed upside-down for your convenience. And so on.)