Whatever happened to units from the 6th and 7th editions of Warhammer, such as the Avenging Knights of the Cleansing Flame, the Death Commandos, and other cool units like them? Are they still available, or have they all been killed or disbanded? Would we be able to hire some of these units?
They are a lot older than 6th and 7th ed, try 3rd and 4th. They were not even called Dogs of War back then. And yes, they have since long been discontinued, although I believe Baumann is planning on updating them.
As far as I know, these units are no longer available for a long time. The Nightmare Legion for example had rules during the second edition, as did the Knights Of the Blazing Flame...they're gone for about 15 years now.
Others of the old Regiment Of Renown would be the Dragon Company, this old Regiment of Gargoyles (don't remember its name) and the Mother Crushers, a regiment of Black Orcs.
With their old rules given and some adjusting you surely could make some rules for them, but official rules, I rather doubt that...
They were the Gargoyles of Barda.
There were also some Wood Elves - Skarlok I believe?
It is unlikely they will release rules for these again, though they may be mentioned in fluff sometime in the future (you never know)...
Yup, Scarloc had a WE RoR.