I like the slavery rule; maybe you tweak it that you can only whip captured cities.
I think civics-wise you should be limited to:
Government: Despotism, Hereditary Rule (Emperor), or Representation
Legal: Any but Free Speech
Labor: Any but Emancipation
Economic: No State Property or Environmentalism
Religious: Any but Pacifism
Your 3 settler limitation is interesting but could cause a lot of difficulty early ... you want to Praetorian rush ASAP, but you couldn't afford the cities you capture ...