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Scenario would be that, for example, Ultramarines are requesting help from the Inquisition. And the Inquisition send them 6 Grey Knights to aid them ( like in DoW game ).
Is this possible or just pure speculation by Relic?

After reading the codex, I get the impression that the chapters don't(for the chapter's pride) or can't request the help of the Inquistion, but infact, the Inquistion just shows up with out being invited. The Inquistion, have different orders for different sitiuations. In the case of GK, they go where ever there are daemons, and alot of them. If the GK do show up while there are space marines or IG on the battleground, then those IG or Battle-brothers are either Killed or brain-wiped because the GK are so secret.
But it would seem practical to let the Inquistion know that there are , Heratics or Daemons in the area.

On request, but they go where they please. All Space Marines bar a select few are mind-wiped. Important command figures are trusted with knowledge of The Grey Knights existence, Dante is an example. The unit in DoW is not fluff oriented, it's a simple game-play addition.

Logan Grimnar called for assistance from the GK during the 1st war for Armageddon...
Admittedly he may have been a bit out of his league with a daemon primarch and all...

Never ask The Inquisition for help, it never turns out well.