As the BrB says on example of a space marine with the Imperium keyword, you can put any unit with the same faction keyword in one detachment.
The detachments from the BRB have only one restriction - faction. It is not nessecary that every unit matches with every faction keyword from their dataslate.
For example, we can take a Brigade detachment and fill it with Harlequine Troop masters leading Grotesques guided by Farseers. Or take Wawe Serpents as Dedicated transports to our Wyches (we can't ride them but still can take).
Is this really so or did I miss something?