To my thinking the best novels of "alternative worlds" generate and provide entire cultures, not merely "localized communities." There are plenty of various examples in history where slavery was part of a long term culture, not always economically based (as in the United States pre-Civil War). In Turkey, for example, slaves were used in the navy (both military and commercial). There were also slaves in the army, which eventually grew into a political power. Eunuch societies, too, often became powerful, even in Christian empires such as at Byzantium. Eunuchs were essentially slaves and did, indeed, engage in sexual activities. The primary thing to consider is how you want to present slavery. The more diverse and well-rounded the presentation the more interesting and creative it can be.