Well that was unexpected....

Monty Python jokes aside, by now most of you have heard of the Codex: Inquisition announcement, and for those who haven't here's what Games Workshop: Digital Editions posted over on the Black Library Blog (emphasis mine):
This November, you will be able to download the brand new Codex: Inquisition.

A full codex, packed with background and artwork on the forces of the Imperium’s most secretive and powerful organization. You’ll soon be able to field an Inquisitor and their retinue in any Imperial army in games of Warhammer 40,000.

But that wasn't all, for they also had a little tid-bit on Facebook when asked how it'll fit in with the rest of the books:
You'll be able to ally this codex with any Imperial army.
you'll need to wait a little while for exact details on how the new book is going to work though.
But that wasn't the only clue we got so far this week because Forgeworld may have given a little clue on how this book may work. First let's look at the Red Hunter's Chapter Tactics to get some basis for where I'm coming from:

 Mnemonic Redaction Protocols: All models with Chapter Tactics (Red Hunters) have the Adamantium Will special rule. In addition, once per game at the beginning of any one player turn, the controlling player may declare that a number of their units with Chapter Tactics (Red Hunters), including Dreadnoughts, equal to the number of the turn being played will have one of the following special rules, chosen by the controlling player, until the beginning of their next turn. For example, if the protocols are activated in Turn 3, then three units may choose a single special rule from the following to apply:
• Counter-attack
• Monster Hunter
• Tank Hunters
• Hatred
• Skyfire
• Interceptor
By this Seal: When using the Allies Matrix, all models in the Red Hunters detachment count units from Codex: Grey Knights and Sisters of Battle as Battle Brothers so long as an Inquisitor is also present in the allied detachment.
Now as most people are aware, and to let those who aren't in on it, currently the Sisters of Battle (aka Codex: Adepta Sororitas) do not have access to an Inquisitor in their army list. When asked about this little detail FW had following to say (emphasis mine):
 The Chapter Tactics update is fully up to date with the current codexes and is in fact future proofed. All we can say is that it is correct and the Red Hunters Chapter Tactics will make sense at a later date.
So what does all of this mean exactly? Well this is where my speculation kicks in and you're free to add your own, or disagree with my assessment in the comments below.

Zion's Speculations

First we know that an army can only take one ally, so for the Red Hunters Chapter Tactics to work and them to ally to a Sisters army with an Inquisitor in it that Inquisitor needs to be in the Primary Detachment. For that to happen the Inquisition needs to not ally as a separate army but more like a supplement or expanded army (or even like Forgeworld does): by modifying the army list.

This idea seems to be supported by the statement that you can field an Inquisitor 'in any Imperial army' instead with 'with any Imperial army'. Naturally though, since English is a mess of a language that could be me reading into it, but for the sake of this speculation we'll treat it as if it were true.

Basically, the army, while completely possible it can be played on it's own but allies into other Imperial armies as if it where a part of that codex, perhaps even with new wargear only available to armies who fit the right criteria (such as to make Deathwatch possible for instance).

Now mind you, this is all just speculation based on a small amount of facts available and we won't know more for at least a week but until we know more that's my theory, that Codex: Inquisition is going to bend some rules regarding allying by working more like a supplement than a separate book.

But enough of my theory, how do you think it works?