I was meaning that the Black Templars would not hang out with 'witches', their name for any one with psychic power. They hate them to the point they dont even have librarians.
In thinking about it I was assuming that that Inquisitor was working for the Ordo Malleus (Daemon Hunters)because they suspected Titus of being posessed.
Ordo Malleus Inquisitors are typically psychic, so the Templars would want to kill them rather than work with them...hence Crimson Fists,called lap dogs of the inquisition for how keen they are to carry out the Inquisitions orders against other Chapters, would have ben the better choice, or in an extreme case Grey Knights, but being even more valuable than a normal space marine being the retinue for an Inquisitor seems a bit below them.
But since the Inquisition could be talking to Titus because they beleive him to be an unsanctioned psyker, and thus somthing Ordo Hereticus would be looking at, then the Templars are a good choice. Providing of course the Inquisitor is not psychic. The templars would be toatally down with a good old fasioned witch hunt, but would not work along side a psyker.