I'm sure the Klingon prison population includes everyone who commits crimes in what they consider their territory. But as a rule, criminals are more likely to commit crimes closer to home, simply as a matter of access and opportunity. Therefore it's logical to conclude that the majority of those who commit crimes within Klingon territory are residents of Klingon territory. So while some
Rura Penthe inmates would be from outside the Empire, it's reasonable to conclude that the majority of species we saw at Rura Penthe are subject races.