I've used skinny for elves before, it didn't take though. I'd use that again, I thought it was a good in world slur.
Dwarves: Grunters, Shinies, Rockhead or Stonehead
Halflings(if they ever come back): Biters
Half-elves: Half-human (if an elf), mistake, unwanted
Mul: Abortion (probably very dangerous to say to a mul)
Human: Breeder (it's close to breed I know), wall-rats, hairies (for a dwarf), soft-skin, kneelers
Elves (too soon?): Tall-ladies (especially to a male elf), twiggies, screamers, stickmen