There is no predominant “slave race” on Fletnern.  People of any ethnicity can be slaves, as can most people of any race (elf, human, dwarf, etc.).  Regionally there are some common situations:

Garnock:  In Garnock, halflings are the predominant slaves, and most halflings in Garnock are slaves.  This is due to the attacks on Villai by Garnock in 616P.  Halflings are seen as strong enough and smart enough to be useful, yet still easy enough to overpower should the need arise.

Myork:  Though slavery is technically legal in Myork, any Angle slave in the city-state will cause alarm amongst law enforcement.  Human slaves of other colors (not Anglic or Tandish) are accepted far more readily, but those holding Anglic slaves will need to be ready to prove their ownership frequently - so frequently that it may not be worth holding them.

Aldar:  The aldar rely on slaves of all classifications, but they are often hesitant to purchase non-aldar slaves.  Living underground as they do, slaves of other races often cannot thrive or even survive away from the sun.  For this reason alone, most of the aldar slaves are also aldar.

Orcs:  The orcs often take other orcs or goblins as slaves, but they do not typically tolerate humans taking orcs as slaves.  The orcs feel they are superior to humans and therefore cannot abide by humans holding their own.  When possible they will raid human communities known to hold orcish slaves, but they are practical about this and will not risk their lives simply to free orcs “stupid enough to have gotten caught”.  While orcs will take non-orcs as slaves, they tend to treat them better than orcish slaves.  Non-orc slaves are seen as more important trophies and therefore the orc will want to keep them healthier so they will last longer.

Dalavar:  For all known history, the Dharvics have raided the Quassim Islands and taken Niko slaves.  They have also raided for Detheb slaves, but the majority were Niko.  This has started to shift slightly since the earthquakes in Lympeid.  

Chuhlme:  When a slave trader acquires a Glodon slave, it is most advantageous to sell such a slave to a Glodon trader.  The Glodons are known for purchasing or ransoming loved ones or any kin.  For this reason, there are very few Glodon slaves outside of Chuhlme.  The Glodons also see Dharvics and Dethebs as dishonest and therefore prefer to have slaves of their own race.  Typically Glodon slaves will be allowed as house slaves and other easier tasks, while any slaves of other races will be put to work in the fields, expecting they will not last many years.

Snobist/Rhum:  While slavery is frowned upon in Rhoric culture, it is seen as morally superior to allowing a child to die.  When Rhorics parents are too poor to feed their children, they will sometimes sell them into slavery, hoping that the child will stand a chance of survival that way.  The main purchasers of slaves in the Rhoric regions are the large plantations.  These massive farms have systems for maintaining the slave populations at share cropper levels as well as giving them a chance to work their way out of slavery.  The system is rigged, and working your way out of slavery is very difficult.