Mechanicum is Mechanicum and not the Inquisition, in the Lore!

They work sometimes together and some Inquisitors have some tech mages as old friends/allies, but a tech mages will never be an Inquisitor.

But in the game, WE will be an Inquisitor.

Definitely think there should be an option for henchmen or to recruit survivors or npc's you meet. Maybe have them have the potential to betray you and actually be a chaos spy to balance them out plus it would add a bit of risk and reward. 

I think a mix of disposable Imperial guard types and/or some more interesting filled out characters that have the chance of dying for good each mission or falling to chaos could be really interesting dynamic. 

I understand the drive as a pet based archetype in ARPGs is definitely an accepted norm and something I would definitely be keen to see. 

Where would this fall against, say, henchmen accessible for all character types? 

Having them passive vs passive / active vs active only would be interesting - a passive only crusader tank for your psyker? a passive ranged stormtrooper who can call down a short cooldown of thunder for your dodgey Assasin? An active only daemonhost / Officio Assasinorum operative who shows up and wrecks face for your Crusader?

there's some interesting design space here - I have no strong preference on implementation. 

PS AdMech as Inquisitors is a weird fluff space - Magos have a lot of freedoms already and Mars is effectively a separate entity under the Imperium in treaty with Terra under the Emperor as Omnissiah. If I recall correctly, the Head Magos has a seat on the High Lords of Terra - the body directly under the Emperor, and with which the Inquisition is effectively equal. As such, what is the draw for a Magos to join the Inquisition if they can get similar freedoms within the Priesthood of Mars without all that pesky Imperial bureaucracy getting in the way... 

Techsorcist with servitor pets could be a option

Or an AdMech Magos running various types of servoskulls and combat servitors - for the more Puritan inclined.

That would be interesting and they could even be an Acolyte or Acolytes, as Inquisitors would sometimes obtain them.