The New Essential Chronology does says this exactly "One of the policies Nadd instituted was to banish criminals outside the walls of Iziz, where they would be devoured by the voracious predators. However, some of these exiles managed to band together and survive, even learning how to capture the beasts and domestitacte them."
Then it continues about how they further advanced, and all that. Now, from the actual source material (Tales of the Jedi):
That was Arca describing the basic history, and although he doesn't mention Nadd (he never says anything about him until the end of the comic anyways), he describes the history perfectly in conjunction with the NEC. Which would lead one to believe that the Beast Riders didn't come into existance until "one of Nadd's policies", therefore Nadd was there before them.
And Lightsnake, Nadd can affect the living:
He floored Vodo across an entire room, yes it said his "hatred lashed out at him", but how can that be if he is powerless?
He instakilled King Ommin, granted he was weak, but nonetheless effected him. He was able to pull Exar Kun's lightsaber away, too.