The "first hurdle" for the new Anglo-American peace plan for Rhodesia will be to escape either a "Soviet or Chinese veto in the United Nations Security Council, Ambassador Andrew Young said yesterday.

American and British negotiators are gambling that Africa's black nationalist leaders are interested enough in the plan to induce the Russians and Chinese to allow its first stage to be launched.

This can be an awkward problem for Moscow and Peking, which are rivals in everything, including championship of African nationalism. China and the United States often support the same black sides in Africa's black-white struggle.

So far in Africa, "Everybody agreed with just enough of the plan to keep it going," Young told the House International Relations Subcommittee on Africa, headed by Rep. Charles C. Diggs, Jr. (D-Mich.)

"Considerable difficulties and uncertainties lie ahead," said Richard M. Moose Jr., assistant secretary of state for African affairs. But he was encouraged because "we have just enough agreement to allow us to move ahead to the next step."

Under the plan made public last week, the U.N. Security Council will be asked to allow the secretary general to appoint a special representative to help arrange a cease-fire in the Rhodesian guerrilla war. A U.N. peace-keeping force would be introduced at a later date.

The U.N. representative would work with British Field Marshal Lord Carver who would be Britain's resident commissioner in producing a peaceful "surrender" of power by Rhodesia in 1965 unilaterally declared independence of British colonial rule.

Smith has labeled portions of the Anglo-American peace plan for bringing majority rule to Rhodesia's 6 million blacks and 270,000 whites as "insane." But he has not formally rejected the seven-point proposal, nor has South Africa, which controls access to Rhodesia's economic lifelines.

In a television interview yesterday (CBS Morning News), Young said "the first hurdle we have to cross" is avoiding a veto in the Security Council when the initial phase of the plan is presented in the next 10 days or so.

The best thing going for the plan in Africa, Young told the House subcommittee, is that "the alternative is a kind of chaos that nobody wants."

At this stage, he said, there is interest in a cease-fire among all sides: the Patriotic Front which conducts the black guerrilla war; the supporting "front line presidents" of nearby nations (Angola, Botswana, Mozambique, Tanzania and Zambia); the black nationalist leaders inside Rhodesia; and the white governments of Rhodesia and South Africa.

But on the overall plan, Young said, "everybody has a qualified participation . . . nobody is giving up their present options at this point."

According to British figures, Young said, "there are somewhere around 800 and so foreign mercenaries" fighting for the Smith government, but "how many are Americans we are not sure."

Young said that in Salisbury, Rhodesia's capital, "there are very few or no signs of strain [but] we have heard that the guerrillas have pretty much free access to the countryside."

He said, "I think essentially Ian Smith wants to internationalize the conflict . . . because he thinks that would rally Western support." But the United States assured him we would not be drawn into that kind of conflict."