In the rule book there is an Inquisitor elite package but no Interrogator elite package.
Assuming that a group of acolytes at some points grows strong enough, and their inquisitor decides that one of the acolytes should be groomed for a future inquisitor, that acolyte should first obtain the rank of Interrogator. I think the period where the players are working towards obtaining the inquisitor rank is quite an interesting one, and do not see any reason to skip directly to inquisitor rank. Interrogator rank seems like a good milestone on this journey.
An idea to handle this as an optional rule, could be to allow the acolyte to buy the Inquisitor elite package, but not be allowed to buy inquisitor talents before being promoted to inquisitor. How does that sound?
Perhaps the influence requirement could be a little lower for the interrogator advancement, but still require 75 influence before the actual promotion to inquisitor can happen.
My main concern is that the leadership aptitude should get awarded at interrogator rank so the player won't hold out on any leadership purchases until inquisitor rank. Leadership stuff is already needed as an interrogator.1000xp might be a lot when not being allowed to buy the talents, but consider it as an investment.