Justified: Robert Quarles, who are those boys he's torturing?

The boys are targets of opportunity who fit Quarles' "habit" (as you put it). The "habit" is that he goes after street hustlers. They can be pimps, prostitutes, addicts, pushers, thieves, or all of the above. They are those who are young, but have chosen a life of ill repute.

We first learn of Quarles' habit when we find out some of his past from this website:

Quarles's father was a heroin addict who pimped Quarles out to other men. Once Detroit crime boss Theo Tonin learned of this horror, he offered Quarles the chance to kill his father, which he did when he was 14 years old. After that, Tonin took Quarles in as a son. Quarles grew to despise his adopted brother Sammy Tonin for his cowardice, and even more so when Theo allowed Sammy to have his inheritance over Quarles. While in Detroit Quarles had a habit of beating male hustlers and even put one into a coma for three weeks. This brought a lot of unwanted attention so Quarles relocated to Harlan County, Kentucky for a new start, getting into the Oxycontin business

In the episode Guy Walks Into a Bar - S03E10 we learn a little bit more about Quarles' youthful troubles (again from this website) -- this is some of the same, rehashed, but reinforces the point:

Donovan, a friend of Brady Hughes (the street hustler that Quarles had bound and gagged in the back room earlier this season) shows up and holds Quarles and Duffy at gunpoint. He believes Quarles is responsible for the death of his friend, Brady. Quarles talks the boy down, explaining that his own father was a heroin addict who pimped him out to other men. When Theo Tonin caught wind of this horror, he offered Quarles the chance to murder his old man and a 14-year-old Quarles did just that. So, Quarles explained, he understood Brady only too well and only wanted to "set him free." Wynn watches in horror as Quarles, crying and shaking, lowers Donovan's gun and embraces the boy.

As Season 3 goes on, Quarles gets addicted to his own product - Oxycontin. Whenever Quarles has a failure, he turns to Oxy. After the initial dose of Oxy, we see him become more and more violent, devolving faster towards his inevitable demise. Because of his new found addiction, his predilection towards violence, and thirst for young men (old boys??), we see these three things come together in a "perfect storm". Towards the end, he really doesn't care about the boys' background, just his wants and desires. As he gets higher and higher (in the intoxicated sense), he becomes lower and lower, taking whomever stands in his way with him. The reason his "tastes" seem to wonder all over the board by the end of the season is because the character is floundering in his addictions and past, self-destructing at an ever increasing pace, leaving the watcher going, Wow, really? and shaking our heads. The character no longer really knows exactly what he wants (we clearly see what he wants in the early episodes of the season), so relies on his sub-human instincts to satisfy his primeval needs. He is all over the place because he has lost his way.