HONG KONG'S Institute of Company Secretaries is giving strong support to Government plans for a thorough review of Hong Kong's Companies Ordinance. Institute chairman John Brewer said that while many other countries treated large public and small private companies as different subjects, Hong Kong's company law followed the British model with one regime governed all companies. Mr Brewer said: ''Our companies ordinance operates to treat the small private company as the exception when it is, in fact, the typical. ''Of the 400,000 or so companies incorporated in Hong Kong only a few hundred are the large, joint-stock corporations which legislators had in mind when drafting early British company law.'' Mr Brewer said similar reviews had been undertaken by former Commonwealth countries, and that Britain also had similar plans. He drew attention to China's new national company law, which included distinctions between the regulation of large public and small private companies. He believed the territory Government should consider the huge amount of work done by mainland authorities, who had extensively reviewed company laws around the world before finally drafting their law. Mr Brewer said that the institute had felt it was unrealistic to subject small owner-managed companies to the detailed filing requirements, high audit standards, and complicated winding-up provisions which applied to large, listed companies.