Scope. All the provisions of this act shall apply to the employees of all boards, commissions and departments of any county in which this act shall be adopted. When adopted in any county, the provisions of this act shall also apply to boards, commissions, or departments of the county constituted by law a body corporate. Any such board, commission, or department of the county constituted by law a body corporate shall be considered as a separate entity in the application of the provisions of this act, and the service of such body corporate shall be separately classified and treated with regard to appointments, promotions and removals, and service records and ratings.