I've been thinking of a way to fold in the Demons from Supernatural, but haven't had any luck.
It seems easy to me.
Demons are just like they are in Supernatural. Don't change anything. They make deals for souls, etc.
There are also Fallen. These are just the way they are in Dresden. Since they most likely don't interact much with demons, and they are mostly stuck on Earth, there's really no conflict. The Fallen can generally beat the crap out of demons, and demons would mostly be terrified of them for that, as well as in awe of them for being the siblings of the Lightbringer.
Outsiders are the way they are in Dresden. Someone who didn't know better might call them demons, but they're not. They're more rare, more frightening, and from somewhere else. Still no conflict.
Buffyverse demons aren't terribly difficult either. They're corporeal monsters from another dimension. People call them demons and the dimension Hell, but it doesn't really matter that they're wrong.
Hellgods are things that rule these other dimensions. With the exception of Glory, they basically never ever come to Earth, so you can hand wave them. They're basically gods of the other dimension If you wanted to include something like Glory, it'd just be a Hellgod bound by powerful magics into flesh and trapped here, so you get to make up the rules of the curse and make a unique creature that's likely in the same rough ballpark as the Fallen.
The Old Ones from Buffy are basically gods. With the exception of Illyria, they're almost entirely dead or gone. Don't worry about them.
That stuff all fits together basically seamlessly. The one place you get a hiccup is DF demons, if you want to make them denizens of Hell. Because we've already got a place we're calling Hell. So just call this Hell a place in the Nevernever, and these demons are basically Nevernever spirits that like to call themselves demons because they know it messes with people.
So we've got "demons" living in three "Hells." One is a corporeal realm beyond the closest layer of the Nevernever, one is an incorporeal realm in the Nevernever, and one is "somewhere else," a place that the dead go when they've sold their souls; this last one seems closest to what a Christian would understand as Hell, but the other two would very likely look the part.
Then you've got malicious spirits, damned souls, and corporeal entities, along with the Fallen, a couple varieties of what amount to gods, and Outsiders. It's a little busy, but compare it with old school D&D with its many outer planes and stuff that lives there.
