I suppose they went 'general' becasue if you stop and look at each and every world, the list would be its own book; I have no doutb they will expand on the origins in later books, but for the moment, I believe the options are wide and different enough to make lots of different regiments.
Worse case, a simple change of an option would work; take the Hive world, change accustomed to crowds to way of the gun (from Inquisitor's handbook) and you got a hive world regiment with a Gunmetal City flavour. No official, but still able to work.
Well, there's a couple of months away so adding more options isn't too hard anyway.
And yeah, taking things from other books works. Its not like some homeworld traits arent straight up from DH anyway.
For representing a Forgeworld, one PC is doing Fortress World with Credo Omnissiah instead of Hated Enemy.
For representing a Voidborn, one PC is doing Hive World. He's replacing Accustomed to Crowds and Hivebound with Ill-Omened and Void Accustomed.