Mmm...that makes sense. I don't know why I didn't think of that last night. Maybe we could do it via UAs?
Technically it shouldn't be a big problem to do it, the problem is to set the parameters in a balanced way, so they aren't too strong or too weak compared to other civs.
But I like the idea of one civ having less cities. We can increase their unhappiness per city significantly, but this needs careful adjusting too, because we need to set the right values of unhappiness per city both when using my happiness system (which is going to be an optional component), and when not using it.