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.

You know, when I saw Feeding Dependency for Tony Stark, a part of me was expecting the dependency to be for booze.

Booze and women. Preferably at the same time.

For some inspiration on a good scenario for zombies, I highly suggest the Buffy the Vampire Slayer episode Dead Man's Party. Lots of dumb, shambling zombies. Magical mask. Built in end-boss. It's great for a zombie situation.

As for rules, instead of stress, give them a threshold you need to overcome. If you don't cause 3 stress in one hit, you do surface damage that isn't even worth tracking. You either do enough to take the thing out, or you do nothing. Of course, that's past whatever armour rating you decide to give them from toughness.

Shields trade away the ability to use two handed weapons for a defensive boost and an off-hand weapon.

Something to think about is that shields are awesome. When people are wandering around with swords and spears and trying to kill you, a shield is something you want. That's why everyone seems to want the things when you look at historic warfare. The Romans used shields to absolutely huge effect. In Europe, until the advent of hefty armour (and after its invention for the not insanely wealthy), shields were everywhere.

The things are also weapons. Rather amazing weapons, actually, if you know how to use them. And I'm not just talking about the ones with spikey bits or dagger blades worked into them. Spartans actually sharpened an edge on their shields and used them to kill people that got inside the range of their spears; I saw something that suggests that just the weight of the things swung correctly was plenty to kill someone or shatter bones, and that's without the sharp edge.

However, one thing to think about is that shields don't magically stop whatever hits them. If someone's swinging a heavy weapon at you and you block squarely with a shield, you can have your arm broken right through the thing, without it breaking. You're probably going to be knocked back somewhat, and you just might end up falling on your ass or getting knocked into something.

At the end of the day, a small bonus to defend or a small armour rating seems the way to go. +1 or armour:1 seems okay for most shields that aren't huge. And of course that only applies to attacks from the front, and it mostly applies either when you know that particular attack is coming or when you're holding it at the ready because you expect some sort of attack. Weapon:1 or 2 also seems reasonable.

What are you trading away for this? The ability to use a two handed weapon like a long sword, a halberd, or an assault rifle.

Yup. High Concept Compel. It covers everything from wizards blowing up technology to faeries being affected by the touch of iron to swamp monsters needing to moisturize their skin constantly. Basically, any little thing that you might want to add a minor sub-system to the game to handle, you instead manage by using a Compel.

Unless there's a really, really good reason for it, I always use this GMing guideline:

Make your character and play it for a bit. If, for whatever reason, you feel that the choices you made were bad after a few sessions, you're free to rework your character however you need. Just make sure to tell me that you plan to do it, and show me the sheet when you're done.

So if we got a couple of sessions or even a couple of stories into a game and one of my players came to me with that situation, my response would be, "Sure. Just tell me what you changed after." I find it much preferable to have a little hiccup like that at the beginning of a game than to get a few months in and have everything explode.

Even if we made it a few months in before these feelings of inappropriate decisions happened, I'm still good with changing things. I'd just want it to be more of an organic change to the character rather than an initial course correction. Which sounds exactly like what you're doing. So I'd want to wait until appropriate Milestones happened, at which point I'd be totally fine with letting you shift things around.

Maybe this is my years playing Vampire: the Masquerade speaking, but I'm pretty fond of after-hours museums for such things. In particular, I'm very fond of some ancient supernatural beings standing by particular exhibits and laughing at their inaccuracies.