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 division of inspection shall not be subject to any civil service rules. Inspectors assigned to the division of inspection shall be designated as building inspectors or as district engineering inspectors according as their duties relate to buildings or engineering. The commissioner shall establish the requirements and qualifications for officers and inspectors appointed pursuant to this section.