If I remember rightly, 6th ed Chaos could have joined armies, but all three factions had their own books. So Dragon Ogres, Harpies and Trolls were probably in the Beasts book, which meant they could show up in normal mortal armies. Ogres may have been dropped from Chaos by that point.
I think the system was something along the lines of: Every Chaos book has units in core, special and rare. You're limited by how many of each type you can take. The primary book you're using fills the relevant slots as normal, but you can take any units from the allied 2 books by moving the slot 'up' a level.
eg. A Chaos Warrior army can take core daemon units, but they take up a special slot, and special daemons, but they take a rare slot. You can't take rare daemons.