There is a file in data folder called something like "civilization file"
You will find this line
<Row CivilizationLevelType="CIVILIZATION_LEVEL_CITY_STATE" CanFoundCities="false" CanAnnexTilesWithCulture="false" CanAnnexTilesWithGold="false" CanAnnexTilesWithReceivedInfluence="true" CanEarnGreatPeople="false" CanGiveInfluence="false" CanReceiveInfluence="true" CanBuildWonders="false" StartingTilesForCity="5"/>
You can change "CanFoundCities" into true. I don't know if they will actually make new settlers and if they do if they would actually use them. But I do know for a fact changing it doesn't crash the game. I just tried it but did not play a full game with it.
There is a simple mod that gives City States different starting units, you can change it manually easily. Again I don't know if they will actually use them, but again I do know the game won't crash.