In the improbable setting of a luxurious Mexican beach resort, an unlikely assemblage of world leaders gathers today for a brief attempt to ease old suspicions and seek new methods of international communication.

President Reagan leaves for the 22-nation Cancun summit on international cooperation and development having said last week he knows he is walking into "a hostile atmosphere" where some want to take from the rich nations and give to the poor.

The bottom line for the poor is food and money. The world now has 500 million chronically malnourished people, described by experts who worry about them as "the marginal people."

The poor nations need $100 billion a year to finance their deficits. Their mounting debt is ever more unmanageable.

Cancun brings together 14 developing nations and eight of the industrialized rich. Per capita income in the participating nations ranges from $90 a year in Bangladesh to $11,930 in Sweden.

The 127 developing nations have three-quarters of the world's population but produce only one-fifth of its product. In the past decade the gap between rich and poor nations has widened.

The meeting brings together widely different viewpoints, with the United States the nation most committed to resisting dramatic changes in the world order, and the poor nations seeking to pry control of international aid and trade institutions from the hands of the rich.

However, the possibility of a major confrontation diminished after it was decided at a preparatory meeting that the summit will have no formal agenda and produce no final communique.

Although Reagan prepared for Cancun by delivering a speech last week telling the nations of the developing world that private investment and free trade are their best routes to greater wealth, the Cancun participants appear to have no desire to spend the two days of formal meetings attempting to isolate the United States.

In recent days one Cancun participant, French President Francois Mitterrand, said there is a risk in such unprecedented multilateral summitry: if no progress is made, there could be a disastrous reaction from the poor nations.

Secretary of State Alexander M. Haig Jr. agreed yesterday that there is a risk. "If the dialogue were totally sterile it would be a setback," he said. "On the other hand, I don't anticipate it will happen."

Haig said Reagan wants to meet with the leaders of all 14 developing nations and may see each government leader in attendance. In these brief encounters, Reagan wants to establish a personal relationship to help future bilateral relations.

Reagan is arriving early today and staying part of Saturday in order to conduct more bilateral meetings than would be possible on Thursday and Friday, the two days of the summit. He will meet Mexican President Jose Lopez Portillo, Chinese Premier Zhao Ziyang, Indian Prime Minister Indira Gandhi, Nigerian President Alhaji Shehu Shagari and Venezuelan President Luis Herrera Campins today, White House communications director David Gergen announced.

The other heads of government attending will be Algeria's Chadli Benjedid, Britain's Margaret Thatcher, Japan's Zenko Suzuki, Saudi Arabia's Prince Fahd, the Philippines' Ferdinand Marcos, Guyana's Forbes Burnham, Tanzania's Julius Nyerere, Sweden's Thorbjorn Falldin, Yugoslavia's Sergej Kraigher and Canada's Pierre Elliott Trudeau.

As described by Haig and other U.S. officials, expectations for results are very low and very vague.

Many of the poor nations want to see the summit make a commitment to "global negotiations," which means that talks on problems of development would be conducted on the one-nation, one-vote principle, giving the poor a majority. The United States and other industrialized nations finessed this at a summit meeting in Ottawa last July. Haig and Reagan said yesterday that the United States would follow the Ottawa position on global negotiations.

Others want to see Cancun give birth to son-of-Cancun, establishing a series of such multilateral summits. Haig said yesterday, "I don't anticipate there will be great enthusiasm for another Cancun." He indicated the fruits of Cancun might be smaller, lower-level meetings on specific issues such as food, trade or energy. Haig's statement appeared to be a step forward by the United States toward the kind of results sought by the poor.

Four leaders--Austrian Chancellor Bruno Kreisky, Brazilian President Joao Baptista de Oliveira Figueiredo, West German Chancellor Helmut Schmidt and President Felix Houphouet-Boigny of the Ivory Coast--are not able to attend for health reasons and are being represented by ministers of their governments. Abdus Sattar, acting president of Bangladesh, is staying home for internal political reasons.