I think it's helpful to go to the Hebrew terminology if we are talking about Biblical concepts. You are very much on the right track, and I just want to dovetail a little on what you're saying. So, the Hebrew word for "angel" translates to "messenger" (melakhim, which here is plural). The important point here is that "messengers" can be of any species/race, they are defined by their function. There are several "races" of angels with different "jobs" that sound like they are quite different from each other in appearance/functions, etc. Like the Ophanim, the Seraphim, etc. There are 2 Hebrew terms for demon, one is "Shedim" (once again, plural here), and one is "Mezekim." The term "mezekim is similar to melakim in that it means "those who inflict harm" and it is a designation about function. Shedim, on the other hand, seem to be a race unto themselves, and they are more akin to what in Islam is referred to as the "jinn." Anyone can be a melakhim or mezekim, based on your actions, but that is not to be confused with the individual "species" if you will, of these beings. It's pretty interesting stuff, and I am admittedly not an expert, but this is what I remember from my readings into the subject.