It doesn't work out that well when you have three dozen soldier jobs that your slaves can do + three enforcer jobs that only your elite species can do and you have to fill up the entire soldier jobs with elites before they'll deign to fit into the enforcers slots.
Regular slaves have a lower priority than free pops for pretty much any worker job that isn't Miner, Farmer or rare resource extractor, and even less if they are Domestic Servitude (which basically only has them fill non-Entertainer jobs if there is literally noone else to do it). This includes Soldier jobs: game will make any unemployed free pops Soldiers before they would use slaves.
Battle Thralls, however, have the single largest priority modifier for soldier jobs, other than enacting Martial Law. Enough that (unless there is a bug) they should kick out anyone not also a Battle Thrall.
