A few ways. Initially I keep them in jail until they're no longer angry at you. Buying the jail upgrades helps, as does having a good jailer. While in jail try abusing them to get their fear up.
Once in your mansion, meet them frequently and make them love you - increases obedience and loyalty. Sex helps if you don't stress them out.
Maid. Uniforms. On. Everyone.
Yes quite easily. All the files that you download are easily edited in Notepad or Notepad++ (or, obviously, any text editor). Starting races is tricky - I think you have to edit at least two locations, and a third if you want a special starting bonus (I think it defaults to the human bonus otherwise). In Notepad++, my literal favourite feature is "find in files" - searches all the files in the folder and helps you figure out what to edit. For instance, search for "demon" and find where demon is listed as part of the list of starting races. Then add Centaur/Slime/Dragonkin and go to town.