Aditional question: Does the Slavery Civic 'stack' with the fixed 25% enslaving-odds of the Balseraph Taskmaster and Courtesan units? So you'd have a 50% chance to enslave if you were using one of those under Slavery Civic?
Oh, and some additional info for the OP: Slaves retain their 'Race', but no other enhancements. That means that Dwarven slaves will have improved work-rate and fast movement in hills, Elven slaves will have reduced work-rate but can build improvements in forests, ect.
Also, the Balseraph can use slaves to build 'Slave Cages' in their Freakshows - one for each of the main races (Human, Elf, Dwarf and Orc) to boost Happiness, Culture and (I think) Great Person Generation. Barbaric, but convenient...