From my experience, you can't choose the starting traits, it's always a roll of the dice. So you just have to use the agent command multiple times, until the trait you want pops up. For the fire mage, you would use 'agent wizard' and just keep rolling. It sucks for races like High Elves, because each time you do it, you get a random wizard type, and then a random starting ability. You can make it the fire mage by specifying that in the agent subtype 'agent wizard wh2_dlc15_hef_mage_fire' but when you do that, you end up with a fire mage that looks like an imperial shadow mage. Many characters spawn with the imperial shadow mage model. And you will still have to re-roll for the starting trait. Notepad, and ctr+C ctr+V are your friend for this.