The head of the department or office in which a position classified under this Division 1 is to be filled shall notify the commission of that fact, and the commission shall certify to the appointing officer the name and address of the candidate standing highest upon the register for the class or grade to which the position belongs. However, in cases of laborers where a choice by competition is impracticable, the commission may provide by its rules that the selections shall be made by lot from among those candidates proved fit by examination, but laborers who have previously been in the service and were removed because their services were no longer required, shall be preferred, and be reinstated before other laborers are given positions, preference being given to those who have had the longest term of service, and laborers in the employ of the municipality on July 1, 1949, who, as of such date, have been employed under temporary authority for 3 years or more or during parts of 3 or more calendar years, shall be preferred also, and shall be placed upon the register for such positions without examination and shall be certified before other laborers are given positions, preference being given to those laborers under temporary authority who have had the longest term of service in such positions. In making such certification, sex shall be disregarded. The appointing officer shall notify the commission of each position to be filled, separately, and shall fill such place by the appointment of the person certified to him or her by the commission therefor. Original appointment shall be on probation for a period not to exceed 6 months to be fixed by the rules but all time spent in attending training schools and seminars, except on-the-job training conducted by local Fire Department personnel, shall be excluded in calculating the probation period; provided that in municipalities with a population of more than 500,000 inhabitants, original appointment to the police department shall be on probation for a period not to exceed 9 months to be fixed by the rules of the department. The commission may strike off names of candidates from the register after they have remained thereon more than 2 years. At or before the expiration of the period of probation, the head of the department or office in which a candidate is employed may, by and with the consent of the commission, discharge him or her upon assigning in writing his or her reason therefor to the commission. If he or she is not then discharged, his or her appointment shall be deemed complete. To prevent the stoppage of public business, or to meet extraordinary exigencies, the head of any department or office may, with the approval of the commission, make temporary appointment to remain in force not exceeding 120 days, and only until regular appointments under the provisions of this Division 1 can be made. In any municipal fire department that employs full-time firefighters and is subject to a collective bargaining agreement, a person who has not qualified for regular appointment under the provisions of this Division 1 shall not be used as a temporary or permanent substitute for classified members of a municipality’s fire department or for regular appointment as a classified member of a municipality’s fire department unless mutually agreed to by the employee’s certified bargaining agent. Such agreement shall be considered a permissive subject of bargaining. Municipal fire departments covered by the changes made by this amendatory Act of the 95th General Assembly that are using non-certificated employees as substitutes immediately prior to the effective date of this amendatory Act of the 95th General Assembly may, by mutual agreement with the certified bargaining agent, continue the existing practice or a modified practice and that agreement shall be considered a permissive subject of bargaining. A home rule unit may not regulate the hiring of temporary or substitute members of the municipality’s fire department in a manner that is inconsistent with this Section. This Section is a limitation under subsection (i) of Section 6 of Article VII of the Illinois Constitution on the concurrent exercise by home rule units of powers and functions exercised by the State.