If Contact/Special Circumstances could be considered
to have a hierarchy, certainly at least outside
of The Culture, I suppose a mixed Navy/Army system
could be used.

i.e.

Referer(s)=
Admiral of the Fleet/General of Ground Forces
or Field Marshall

General Systems Vehicle=
Vice Admiral

Culture 'Mercenary'=
Major General

General Contact Unit=
Commander

SC Operative=
Lieutenant Commander/Major

Culture Warship (ROUs etc)=
Lieutenant/Captain

Superlifter=
Lieutenant (Junior Grade)/Second Lieutenant

Module=
Ensign

SC Drone=
Gunnery Sergeant

GCU crewman (Human)=
Petty Officer First Class

GSV crewman (Human)=
Petty Officer Second Class

Contact Drone=
Petty Officer Third Class/Corporal

Ship Drone=
Crewman/Private