Well, most people from what I've seen have been using two sources.
The "CAD" or allies detachments are the only ones that give you the standard advantages, like objective secured and re-rolls to any warlord trait roll.
Otherwise, the detachment units do not have obj secured and can be wildly different.
The Hellbrute detachment, for example, don't count as elite or troops, but instead as their own units that can capture objectives, but don't get the objective secured advantage.
So if you play Chaos, you can use CSM and a Hellbrute detachment as your two sources.
I would talk with your group though, as opinions do vary...