Oh I see, that's what I didn't understand. The fiducial model is embedded in the robot model in the .world file, as well as the "provides [fiducial]" is declared under each of the robots driver ; or is the model of the fiducial declared parallel to the robots in the .world file, and one "stage driver" providing a fiducial interface declared in the .cfg?
In either case I don't see where I define the id of each of the fiducial markers and etc. If you could post any simple .world and .cfg file you already have so I could learn from it it would be great . Anyways, I appreciate your help very much, sorry if I'm bothering you too much.