President Carter buttressed the authority of his incoming Secretary of state, Edmund S. Muskie, last weekend at Camp David but also made clear that Zbigniew Brzezinski will continue to play a key role in the formulation and public expression of foreign policy, administration sources said yesterday.

The president's remarks, in lengthy meetings with Muskie and other senior foreign policy figures, were received with cautious optimism by State Department officials hoping for an end to internecine conflicts within the administration.

Carter's remarks made clear that he expects Muskie to take the lead in articulation of the administration's most basic policies to the U.S. public as well as to foreign governments, accoroding to the officials. This was interpreted as an endorsement of Muskie's claim that he will be the number one spokesman, next to the president, on American foreign policy.

However, officials noted that much the same thing was said of Muskie's predecessor, Cyrus R. Vance, on several occasions. Thus the practical impact of Carter's backing for Muskie remains to be seen in the implementation of the general guidelines Carter is reported to have laid down. i

The president is reported to be seeking to return to a more orderly and less crisis-dominated policymaking process that of the past six months, since the American hostages were seized in Tehran. But changes in this direction are yet to be confirmed by specific directives and implementation in practice, sources said.

Muskie met yesterday at his Senate office with Lord Carrington, the British foreign secretary, and was reported to be working on the presentation of his views to the Senate Foreign Relations Committee hearing on his confirmation as secretary of state, scheduled for tomorrow.

If Muskie is confirmed quickly by the Senate, he plans to fly to Europe next week to meet with NATO allies and attend the 25th anniversary of the Austrian State Treaty in Vienna, State Department spokesman Hodding Carter confirmed.

However, the State Department was cautious about a Vienna meeting with Soviet Foreign Minister Andrei A. Gromyko, who will also be there.

Responding to news reports late last week that such a Muskie-Gromyko meeting has been authorized and is likely, officials pointed out that the Russians have not yet agreed to such a session. Moreover, there was concern that the news reports, before actual agreement about such a session, could make a meeting less likely.

In any case, a Muskie-Gromyko meeting in Vienna is not likely to be a lengthy one as the schedules of the two men are now projected. Gromyko is expected to arrive in Vienna around noon May 16 from a Warsaw Pact observance in Poland. Muskie is expected to leave Vienna the same afternoon for home in order to attend graduation ceremonies of his daughter, Martha, at the University of Maine the following day.

As Muskie prepared to address the question of his relationship to Brezezinski at the Senate Foreign Relations Committee hearing, the committee made available correspondence from several former high officials opposing a high visibility role for the White House national security affairs adviser.

Former secretary of state Dean Rusk said the presidential foreign policy adviser should not create "a mini-Foreign Office" in the White House, should engage in negotiations with foreign governments "in only the most exceptional cases" and should not have a press spokesman or major public role.

McGeorge Bundy, who was national security affairs adviser to President Kennedy, called the post "an inside, not an outside, job." Bundy said it is "highly important that this individual should not become a major public exponent" because of the danger of confusion.

Henry A. Kissinger, who served as White House adviser and later secretary of state, said in a Time magazine interview that the White House aide should not be perceived as one of the chief originators of policy, should not appear on television or see foreign diplomats due to competition with the secretary of state. He added that he had violated these precepts in the office but now found such action "unsound."