As I understand it a good portion were part of the Allied Mercenary Command (AMC) which were wiped during the Jihad. Sarna has a listing, presumably the larger formations. As many were lance to company sized when founded. AMC being destroyed was likely some house cleaning. However, I know of no official listings prior to the Jihad.
As too their missions, no elaboration, but likely a way for players to have their own Dragoon adventures. Also, a plausible explanation as to where the Dragoons got their replacements prior to their orphan and breeding programs (pre-invasion).
Edit: However, looking at Sarna some of the units listed would pre-date the Dragoon's arrival, so maybe not a good list. The most famous unit would be Miller's Marauders/Barber's Marauder IIs if the original lore still stands, and they are not even listed...