The steady stream of Cuban exiles into south Florida demonstrates again America's ambivalence toward the newcomer: the open arms of the nation known as the world's melting pot, the wary eye of a people who want to protect their own good fortune.

Throughout its history, the United States has been beset by periodic waves of bigotry and cries of alarms about batches of newcomers with strange-sounding names and tongues.

There were discriminatory immigration laws for the first half of this century. Even the waves of refugees since World War II have been greeted with suspicion as well as smiles. But in the long run, the record shows, America has assimilated its new masses.

Experts agree that the United States will always be a magnet for economic migrants seeking a better life as well as for the growing numbers of refugees -- 16 million at last count -- forced from their homelands by the continuing instability of world governments.

The contradictory goals of the Carter administration in facing the current crisis reflect the same historical ambivalence. "We can't afford to take them all in. We can't keep them out," was how refugee coordinator Victor Palmieri characterized the Cuban exodus in an interview a week ago.

Yet even this past week, while the nation's attention was trained on the emotional story of the Cuban exiles, Indochinese refugees by the thousands arrived in Oakland for resettlement, as did hundreds of Soviet Jews landing in New York City after flights from Rome.

They are part of more than 230,000 "refugees" the United States already has committed itself to accepting this fiscal year. That's in addition to 290,000 coming in as legal "immigrants." The total number of newcomers increases still more because in recent years up to 125,000 immediate relatives have been allowed to enter without being counted in numerical quotas.

There has been no great national hue and cry about the Cuban migration because it is controlled and orderly. The U.S. government is making the selection, not Fidel Castro.

All of a sudden in the last few weeks, though, the United States has become this year's Malaysia -- the country of first asylum for the world's newest boat people. And Carter administration planners admit they face policy choices with built-in contradictions.

"It's as tough a social problem as we've ever seen," a senior White House official said in an interview. It's complicated, he said, by the sagging economy and the potential local backlash in south Florida.

After initial pronouncements that the administration was taking a hard line and wouldn't accept the influx of Cubans, President Carter seemed to be sending a different signal early last week when he said we'd welcome the newcomers with "an open heart and open arms."

Is this what we really mean to say? "Yes and no," said an administration planner. "If we're going to take people, we will treat them as we should. But that is not to say we want to take unlimited numbers. That's the paradox."

The fundamental problem, this official said, is that Castro hasn't agreed to an orderly way of handling the flow of exiles. "It's not that we don't have a policy. It's just that the most desirable policy isn't available."

The prospect of spending hundreds of millions of dollars to aid the new Cubans while American auto workers are being laid off isn't a happy one for planners.

The immediate major concern of federal officials grappling with the issue is the fiscal impact of calling the Cubans "refugees" as President Carter did last week. For under the new Refugee Act of 1980, such persons are entitled to a range of benefits paid for entirely by the federal government. That's now estimated at about $1,600 per individual on top of the millions already spent for temporary shelter, food and transportation.

An even more important issue, several officials, agree, is how to deal fairly too with the thousands of Haitians who have arrived illegally over the past several years "without opening ourselves up to an unresolvable migration situation," as one put it.

The fear, as another official put it, is that if the gates are thrown open to undocumented workers like the Haitians, the beaches may be invaded by economic refugees from any country within sailing distance.

Thus for now, Cubans and Haitians alike are being processed as applicants for asylum. To be approved, they have to prove a well-founded fear of facing persecution if returned to their home.

Such cases are processed individually at the State Department by only two people, and the average annual caseload of more than 2,000 already is backed up by pending applications from several thousand Haitians and 1,500 Iranians, according to David Martin of State's human rights office.

Palmieri, the department's coordinator for refugee affairs, has said it would take years to handle the Cubans' applications. Therefore, some kind of group determination seems likely.

"Asylum is for Solzhenitsyn and an occasional ballerina," an official said.

No matter how they're processed, the newcomers eventually should blend into American society, according to Barry Chiswick, a research professor of economics at the University of Illinois at Chicago.

Chiswick said in a phone interview that his research into the work patterns of American immigrants shows, not surprisingly, that income is low and the unemployment rate high for the first few years. But after 10 to 15 years they historically have matched and often surpassed the earning power of the native born, he said.

Regular immigrants, whom Chiswick refers to as "economic migrants," do better than "refugees" because they have planned the move and usually have transferable skills. Refugees are forced to move and therefore are not as ready for their new environment, he said.

The first waves of Cubans, 800,000 in the 1960s and of Indochinese, 150,000 in 1975, were unusual in that so many were well-educated.

The second wave of Indochinese, 170,000 since late 1978, were pushed from Vietnam by the Communist government, much like some of the current group of Cubans.

The Hmong tribesmen from Laos have been of special concern because many can't read or write their own language and "are virtually unexposed to western culture," according to a Library of Congress report on resettlement.

The private volunteer agencies that resettle most of the current inflow under State Department contracts are optimistic about America's ability to cope with the new Cuban boat people.

George Wagner, assistant director of refugee services for the U.S. Catholic Conference in Washington, which is resettling more than 40 percent of the new Indochinese, said there are still plenty of entry-level jobs for refugees. "Many service jobs go begging," he said.

The administration's treatment of the Cubans and the Haitians will say something about "what kind of nation we're going to be", he added. "If we're going to turn off the spigot, it's going to change the nature of the country."

Gaynor Jacobson, executive vice president of the Hebrew Immigrant Aid Society in New York, which resettled almost 29,000 Soviet Jews last year, said his group is ready to help resettle the new Cubans too.

"There are always crackpot groups who say we can't take any more people," he said. "But the generosity of the average American hasn't gotten enough attention."

Though Carter doubled the intake of Indochinese to 14,000 a month just last summer, "we haven't got enough Indochinese for all the churches and synagogues who want to sponsor them," Jacobson said.