Question 1, It is late for me to respond to it. As It is a complex system. Adding PET and COMMON_DOMESTIC will let you have elven pets, but not labor. Since only MAIN_RACE creatures in fortress mode and do labors, at least without the aid of DFusion.
Slaves are only possible if they are a caste of DWARF
alter <df dir>\data\init\world_gen.txt and alter the embark points in the those (I think create a world looks for the very top of each size)
Mem hack (programs like: cheat engine, t-search, art-money)
or as you said custom worlds.