you can get just 1 servitor with plasma cannon.
As for a Plasma Inquisitor i like this:
Inquisitor Lord
-Plasma Pistol
-Bolter-Plasma Gun
Targeter
Auspex
*Hammerhands
Retinue:
Veteran Guardsmen with Plasma Gun
Veteran Guardsmen with Plasma Gun
Veteran Guardsmen with Plasma Gun
Acolyte with Bolter-Plasma Gun
Acolyte with Bolter-Plasma Gun
Acolyte with Bolter-Plasma Gun
Sage
Sage
This makes 7 plasma guns and 1 plasma pistol, mount them in a chimera with heavy bolters and extra armour and is a Marine killer inquisitor squad.
Well some of are own guy will get toasted but the Inquisition cares not.
As for the shooty inquisitor:
Inquisitor w/ psycannon
Retinue
2 sages, mystic, and 3 gun serv w/ heavy bolters
you can change 1 of the heavy bolters for 1 plasma cannon if you want.
this makes them more reliable in th shooting phase plus the extra BS for the inquisitor