Section 6. The commissioner may appoint, transfer and remove inspectors, experts, clerks and other assistants. Except as provided in section two of chapter one hundred and forty-seven, the number of inspectors heretofore authorized by law may be increased only with the approval of the governor and council. District engineering inspectors in the office of public safety and inspections shall not be subject to any civil service rules. Inspectors assigned to the office shall be designated as building inspectors, elevator inspectors, or as district engineering inspectors according as their duties relate to buildings, elevators, or engineering. The commissioner shall establish the requirements and qualifications for officers and inspectors appointed pursuant to this section.