Not a Terraria genius, so sorry if these NPCS can't be obtained this way. Mostly explained from title, I have four qualified houses, and I defeated Skeletron, and still have not received clothier or dryad. Again, sorry if I'm wrong.
According to the wiki, these are the NPCs that will occupy your house a certian event.
If any bosses are defeated in that world, the Dryad will occupy your house if it is vacant.