That is impossible. You only ever make slave armies after you have the tech and processing capacity to put neural Implants into thier brains. All thier brains. (wich is still a whole lot less then the entire population of a planet).
All slaveholders are way aware that slave armies would be dumb idea otherwise
Slave armies existed in history, and they were not made with neural implants but by raising the particular slaves from early childhood in the barracks (and no I don't mean the Spartans, who were raising their citizens that way to control the helots).
The reason for it was as an alternative for the monarch to relying on troops loyal to lower level leaders that might not be loyal to him.
I can't think of an example where they sided with a slave revolt, but in one case took over the government and ruled the country for centuries (Egyptian Mamluks), and in another formed an independent power block in the internal politics of the state (Ottoman Janissaries). So, yes, a bad idea in the big picture but not without precedent.