I'd already thought of some of those, but there are a few I hadn't considered. Getting a bit of a discussion going doesn't sound like a bad idea to me, of potential mini types or just Sabriel and the rest in general. In fact, I think I remember a conversation just a bit further down this Board where people were saying it’d be nice to have these sorts of discussions.
I'm currently in the process of re-reading the books while I make notes about the fic, so I'm familiar with some bits, and doing the rest from memory for the moment.
As for the minis:
Greater Dead
My understanding is that ‘Greater Dead’ is just a term applied to anything which is able to return from beyond one of the further Gates under its own power (rather than following in the wake of something more powerful) is considered to be in this category. I think that Chlorr of the Mask and Hedge are both Greater Dead (although Hedge may just be a powerful Free Magic sorcerer and necromancer), and Kerrigor, obviously, but he takes a form of his own choosing after leaving physical body, and then ends up trapped in a form similar to the one Mogget uses most often. I think there is also at least one that Sabriel encounters in Death, described as a cross between a snake and a warthog or something like that? Although I don’t recall that actually being given a name, just how some aspect of it was indicative of it coming from deeper within Death.
I haven’t actually read about any zombie-minis so far, although I am aware that there are loads of missions I haven’t got to yet. I wasn’t sure about using anything Dead, despite it being a big part of the setting, because I thought one of the defining characteristics of minis was being ‘Cute but Dangerous’. Anything with... y’know, actual rotting flesh, is going to struggle to be in any way ‘cute’ (in my opinion).
Orannis
As you say, while definitely a very powerful adversary from the books, it is still just a sphere of metal. Could be used for a bit of humour as you’ve suggested, although unless they get given extra powers/abilities (which doesn’t seem like a good idea) it would be difficult to see them helping out keep order at an OFU. I’m not actually planning on writing an Abhorsen OFU myself, but I figure if someone else is going to be stuck with my choice, I should try and make it a good one for them.
Free Magic Elementals
Yeah, that’s a bit of a mouthful for a name. There are various types mentioned, so a specific free magic elemental could be used, although I think some of them are just named in passing rather than being given full descriptions. The only ones I can recall fully are the Stilken that Lireal encounters in the Clayr’s Library, and the Hrule that Nick encounters in the short story collection (although I’m not completely sure about the classification of that creature).
The Bells
The Bells are definitely important enough to be considered, although they suffer from some of the same potential problems as a mini-Orannis in that they are inanimate objects. Most of the staff of an OFU for this continuum would also probably have access to their own Bells, so having a bunch of independently willed ones floating around might be a bit pointless. It also might be hard to tell apart a mini-Bell from one of the Bells taken as loot (as I would imagine that a large portion of OCs are going to be either necromancers or Abhorsens, and their Bells would make for very shiny trophies).
Paperwing
This is one that I hadn’t thought of at all, but it seems like it could be really fun. They’re a good choice because it’s nice and distinctive, and they are described as having at least a limited intelligence. I also like the idea of a horde of screaming fangirls being dive-bombed by a… I don’t actually know what the collective term would be, a flock? A squadron? Anyway, a horde of screaming fangirls being dive-bombed a bunch of mini-Paperwings, to keep them away from their LOs. And the minis don’t necessarily have to come from monsters, as evidenced by the mini-LEOs from NCIS (although admittedly this may be due in part to the relative lack of actual monsters from that continuum).
Mordicant
The mordicant is another good choice, as it is one of the few recurring threats. A lot of the other weird and wonderful creatures normally only get a scene or two. Although again, I'm not sure it’s correctly placed on the Cute vs. Dangerous scale.
Necromancer
Another key adversary from the series, although mini-necromancer is quite generic sounding. There are plenty of other settings that involve necromancy. And while the Abhorsenverse necromancers will look very distinctive due to their Bells, by the time they’ve been shrunk down to mini size (about 2 feet tall I believe for most humanoid minis?), that distinguishing feature is going to be difficult to pick out unless they’re actually using them at the time.
Charter Sending
One that I’ve just thought of actually, but could be doable. There are various different types that have been mentioned, so they’d definitely be able to help out the teaching staff on an OFU if there ever is one, and they have been shown to have good combat skills, so there’d be no problems keeping the fangirls in check. It just seems... I dunno, a bit boring?
Mogget
Or mini-Bound Bright Shiners (which is far too much of a mouthful for me). Wasn’t sure about using an actual character name for the mini, although I guess there is a precedent for it with Harry Potter having mini-Aragogs rather than mini-Acromantulas. What with them having one of the Bells around their necks, mini-Moggets would have the same humour potential for mucking around with agents as was suggested for the Bells themselves. In fact, if the kittenish aspects get played up, they could even rival mini-Discords for mischievousness.
I think at the minute I'm torn between mini-Moggets and mini-Paperwings for this.
As for the series in general, the things that I like best about it are the unique (and detailed) magic systems, and the fact that while everyone is clearly on the side of Good or Evil, the good guys never come off as preachy or holier-than-thou annoying to make me want to root for the bad guys out of spite.
Obviously there’s more to it than that, but I’ve already turned this into a bit of a monster of a post, so I'm going to shut up now.