Iím just quoting from BFG and the few Battle Fleet based books.
They call them Arms Men, and it has to do with exactly what you say Men at Arms.
In my army itís based on a Black Ship and itís the force used to secure rogue psykers to bring for the Inquisition.
I have a Command Squad with a HSO, Independent Commissar, the 4 guardsmen with medic, standard bearer, and all of have storm bolters, I also took a Psyker and gave him the Honorifica Imperialis Mundanus so heís worth something carapace armor, and a force weapon, as well as a priest with an eviscerator and holy relic.
I have an Inquisitor and retinue, and I have a full pair of platoons and storm trooper (Inquisitorial) as well as a 5 man Ogryn squad led by a Commissar.
I didnít take Heavy Support and I made them all Light Infantry that could be.
The Command Squad, storm troopers, ogryns, and Inquisitor have Chimeras.
I made up for the lack of real tanks with tons of heavy weapons.
"Oh it's you again...." *Bang
Inquistor Arkturus Palov as he banishes a daemon to the warp.