I get the sinking feeling this may have been asked a billion times before, but here goes: the Rogue Trader corebook mentions that rules for buying retainers are in Chapter V: Armoury, but I can't find any there. Will they be covered instead in Battlefleet Koronus? Thanks.

Look at page 272 for the big Acquisition Modifiers chart, then go back to 111 for availability and quality modifiers.

They are basically an item to be purchased, like a lasgun.

But how does one determine what their skill rating would be? I would assume you'd give them a generic skill rating, like Crew (competent, Crack, Veteran Elite).

Razorlight said:

Competent = Common Quality

Crack = Good Quality

Veteran Elite = Best Quality

As to rules being in Battlefleet Koronus, yep it was talked about in one of the designer diaries.