Except as provided in paragraphs (3) and (5) of this subsection, there may not be more than three units in a county and a unit may not include both supervisory and nonsupervisory employees. If a county has more than three recognized units and, as of July 1, 1974, the units have exclusive representation for collective negotiations, these units may continue as negotiating units. In Baltimore County, there shall be three units, including one unit of supervisory employees as defined in § 6–501(i) of this subtitle. In Carroll County, beginning on October 1, 2007: There shall be no more than three units; and All units shall be nonsupervisory units. In Baltimore City, the public school employer may designate a fourth unit composed of all Baltimore City school police officers, as defined in § 4–318 of this article, up to and including the rank of lieutenant. All eligible public school employees shall: Be included in one of these units; and Have the rights granted in this subtitle.