The Exorcists are a pretty poor example, though, because Guilliman left no fixed size for the Scout Company and it could be argued (by some, anyway) that the Exorcists' three Scout Companies all fall under the "Tenth Company" heading.
You'd have done better mentioning the Space Wolves, who have either twelve or thirteen depending on who you ask.
Actually the exact wording in the Space Marine codex under the description of the Exorcists states "The Exorcists maintain two additional scout companies for a total of twelve companies in all". That pretty much means those who would argue they have one really big scout company would be wrong at least per the description of the chapter in the codex. Guilliman didn't but Matt "I'm a DB" Ward pretty much did for whatever that's worth.
I'm aware of that, but A) this is
Matt Ward we're talking about, B) although the Exorcists are themselves a bit weird, the only real organisational
difference from the Codex that we know of is in having multiple Scout Companies, which might cause other Chapters to turn their heads, but -- owing to an allowance for fluidity in Scout numbers -- wouldn't necessarily be "odd" in and of itself.
Also the space wolves are not a codex chapter.
That's more or less my point, although I didn't want to get into it too deeply myself because, well, can-o'-worms time!
The point here is if your going non Codex you can organize your Space Marines anyway you want and there is nothing that says you can't have a non Codex chapter if your using the SM Codex as the rules base.
Well, the OP didn't mention Codex or non-Codex. The impression I got was that he'd be emulating Astartes force organisation in his Guard army, hence the distinction between "standard Codex" organisation and the "notable exceptions". On that note, good call in mentioning the Dark Angels a couple of posts ago.