Section 21. The sergeant-at-arms shall, immediately after his election, with the approval of the presiding officers of the two branches of the general court, appoint such employees as are necessary to fill any existing vacancies in the employees provided for by section eighteen; and no employee provided for by said section, excepting pages, shall be removed except with the consent of the presiding officer of the branch to which he is assigned or of both branches if he is assigned to both. The sergeant-at-arms may appoint and remove such additional clerical and other assistants as the duties of his office may require, and, subject to the approval of the committees on rules of the two branches acting concurrently, may fix their compensation.