The rules as written have their quirks, basically. Some people really hate the d666 mechanic, for good reasons and bad ones, while others find that it really works for them, and still others come to like the d666 when they get the hang of it.
Of all the rules, the one that I would suggest you consider house-ruling is Disturbance. The others, at least so far as I recall, do work for most celestials who are not Jack Of All Trades.
(Celestials need to focus; a 3/3/3 Force-build, with 6 in each characteristic, is kind of ineffective. I should know. I've played it. A 1/2/3 (or 2/3/1, or whatever) build is much better, and at least, characteristics should be 5/7 instead of 6/6. This gives two things: better chances of success, AND, more importantly, a reason for PCs to stick together. They
can't all be good at everything. They
must specialize, and thus must make and keep allies who are good at the things they're bad at. This will be an invaluable mindset if your players want to play demons at all "straight.")
So start off with few or no house rules (again, except Disturbance; unless you like math, winging Disturbance is pretty much universal), and see how things work.
The Superiors PDFs... Well, check out the samples of each, I'd say. O:> They all include the Index, which is very, very useful. For instance,
http://e23.sjgames.com/media/SJG37-3313_preview.pdf tells you that you'll have all the stuff from the core rules (so you don't have to flip back and forth between the core rules and the PDF/printout), plus the more detailed information about the Prince that (for Azzie) appeared in Heaven & Hell,
plus stuff drawn from H&H about Hades (as well as original material),
plus the whole "Players and Pawns" thing, which is about
playing a Servitor of the Game. The mindset, the factions, the tactics, an NPC or two to interact with or use as an example for character creation. Then there's a small selection of adventure seeds that you can spring on players who include (or are all!) Servitors of the Game.
Some of the Superiors PDFs -- Eli, Litheroy, and Zadkiel spring to mind -- were never expanded in the Revelations Cycle at all. They duplicate the material from the original appearance (well, Zadkiel kind of got a complete makeover...), but everything else is Brand New Material designed to help a GM play the Superior and NPC Servitors, and help a player get into the mindset of the Servitors of that Word.
Does that help? O:>