"Middle Class" here meaning "A merchant or landowner who isn't particularly rich", not "3rd and 4th wealth quintiles" the way it's used today. In the sense that we would recognize it, that of skilled or semiskilled professionals earning significant wages for their labor, there would have been only a very small middle class, and most of them would have been, well, landowners in addition anyway. They almost certainly would've been in the 5th income quintile. And probably a multi-generational household as well.
Depends on the household! Some houses had dedicated dormitories or even (small) individual rooms for their slaves, others would simply have the slaves sleep on the floor of the bedroom, or the floor of some other room.