Section 19.  Nothing in Part I of this Article authorizing cities or other political subdivisions to be placed under the provisions of said Part by election, act of the legislature, or ordinance of the local governing authority shall authorize the inclusion in a city civil service system of firemen and policemen in any municipality having a population greater than thirteen thousand but fewer than four hundred thousand and operating a regularly paid fire and municipal police department or in any parish or fire protection district operating a regularly paid fire department.  Such firemen and policemen are expressly excluded from any such system.