God-Emperor
Deity
It is pretty easy to add extra civs into the game at the start of the game.
The troublesome part is to differeniate them from the real civs.
To limit them to only the starting city, you could give them civilization types which are set up to block them from building settlers (give them a UU type of NONE for the settler's unit class, just like the Barbarians have for this). Well, they could also capture cities from other players, but I think the city states in Civ V can do that and I assume that the Civ V sort of city state is what was wanted.
In all other ways they would act as a regular civ. To get more specialized behavior, like the different types of city states in Civ V, would be a bit trickier.