Why do you think Turkey, Nippers and Ginger Nut are introduced to the reader before Bartleby?

It can be argued that Turkey, Nippers, and Ginger Nut are introduced to readers before Bartleby for two key reasons.
First, the author wants to demonstrate why the narrator is initially drawn to Bartleby and believes he will have a soothing influence on Nippers and Turkey.

After a few words touching his qualifications, I engaged him, glad to have among my corps of copyists a man of so singularly sedate an aspect, which I thought might operate beneficially upon the flighty temper of Turkey, and the fiery one of Nippers.

Essentially, the contrast between the three employees and Bartleby could not be more stark. Turkey is known by his predilection for alcohol and has the tendency to turn in abysmal work in the afternoons, presumably after having imbibed large quantities of strong drink. Meanwhile, Nippers usually turns in excellent work but, due to his digestive problems, is prone to fits of irritability and "nervous testiness." Ginger Nut, only twelve, functions as the office's delivery boy, the chief "cake and apple purveyor for Turkey and Nippers."
The narrator hires Bartleby because of his soothing demeanor, not realizing Bartleby's deceptively calm manner hides a surprising intransigence. 
Second, by omitting the real names of the three characters, the author emphasizes how an increasingly mechanized and industrialized society dehumanizes the average worker. Turkey, Nippers, and Ginger Nut are known only by their nicknames. Additionally, they have been reduced to caricatures of masculinity; one prone to drunken fits of emotion, another to obsessive-compulsive behavior, and the third to obsequious behavior before his superiors.