SIX political parties are to meet soon in an effort to reach agreement on the proposed retirement protection scheme. United Democrat member Lau Chin-shek said the unprecedented meeting aimed to strike a common ground for the six parties, which now back two proposals. The United Democrats, the Liberal Party and Meeting Point have endorsed a proposal asking for an immediate old age pension, with a means test mechanism, to those over 65. The three parties also called for the setting up of a central provident fund. In a separate move, the three ''smaller'' parties - the Association for Democracy and People's Livelihood, the Hong Kong Democratic Foundation and the pro-China Democratic Alliance for the Betterment of Hong Kong - have formed a coalition to garner support for an old age pension scheme contributed to by the Government, employers and employees. The Government announced last December that it would embark on a study to prepare for the old age pension scheme. Yesterday, Jimmy McGregor of the Hong Kong Democratic Foundation said he would try to convince the other parties to drop the idea of a central provident fund. ''They know now that there is no possibility that the Government would move ahead with the central provident fund.'' Mr McGregor said he would try to persuade the three parties to make the old age pension a top priority and, as a second stage, push for a compulsory provident fund. But Lau Chin-shek said the six parties would seek common ground on what should be included in a comprehensive retirement protection scheme.