Codex inquisition still uses your allies slot.
They were written as "battle brother allies" in witch hunters and grey knights before the allies rules were actually implemented an edition later. Now they'll play that way, requiring a troop etc
The difference now though is that the troop choice can be the retinue, full of fun stuff, or the retinue can be an HQ choice without taking a slot so that you can take storm troopers.
It's not a full codex in a sense, you'll find fast attack and heavy support a little light on options.
It's the first of what will be ally intended codexes which as I explained about a year ago can stand alone, but not with the same breadth as other armies.
Also, immediately afterward we'll be seeing the Forgeworld forces of the space marines and inquisition to update all those rules as well.