Why were the Chinese allied with the United States in "Red Dawn"

After some history reading I actually have an answer for this. And I'm going to let Wikipedia do most of the talking.

US and China were allies in 1984

The Shanghai Communiqué

The Joint Communiqué of the United States of America and the People's Republic of China, also known as the Shanghai Communiqué (1972), was an important diplomatic document issued by the United States of America and the People's Republic of China on February 28, 1972 during President Richard Nixon's visit to China. The document pledged that it was in the interest of all nations for the United States and China to work towards the normalization of their relations, although this would not occur until the Joint Communiqué on the Establishment of Diplomatic Relations seven years later.

The US and China also agreed that neither they nor any other power should "seek hegemony in the Asia-Pacific region". This was of particular importance to China, who shared a militarized border with the Soviet Union.

The communiqué included wishes to expand the economic and cultural contacts between the two nations, although no concrete steps were mentioned.

The "normalization" referred to actually didn't start to occur until 1979, and by 1984 (which is when the film is set) it was in full effect and the US and China were allies.

Russia and China were not allies in 1984

You base your question on the idea that because Russia and China are both Communist countries, that means they are allies. This is far from the truth.

There has actually been strong animosity between the two countries over the leadership of world communism. This is known as the Sino-Soviet Split

The Sino-Soviet split (1960–89) was the deterioration of political and ideological relations between the neighboring states of the People's Republic of China (PRC) and the Union of Soviet Socialist Republics (USSR) during the Cold War. In the 1960s, China and the Soviet Union were the two largest communist states in the world. The doctrinal divergence derived from Chinese and Soviet national interests, and from the régimes' different interpretations of Marxism–Leninism.

In the 1950s and the 1960s, ideological debate between the communist parties of the USSR and China also concerned the possibility of peaceful coexistence with the capitalist West. Yet, to the Chinese public, Mao Zedong proposed a belligerent attitude towards capitalist countries, an initial rejection of peaceful coexistence, which he perceived as Marxist revisionism from the Soviet Union.

Furthermore, since 1956 (when Nikita Khrushchev denounced the legacy of Stalin), China and the USSR had progressively diverged about Marxist ideology, and, by 1961, when the doctrinal differences proved intractable, the Communist Party of China formally denounced the Soviet variety of communism as a product of "Revisionist Traitors".

The split concerned the leadership of world communism. The USSR had a network of communist parties it supported; China now created its own rival network to battle it out for local control of the left in numerous countries. Lorenz M. Lüthi argues:

"The Sino-Soviet split was one of the key events of the Cold War, equal in importance to the construction of the Berlin Wall, the Cuban Missile Crisis, the Second Vietnam War, and Sino-American rapprochement. The split helped to determine the framework of the Second Cold War in general, and influenced the course of the Second Vietnam War in particular."

The divide fractured the international communist movement at the time and opened the way for the warming of relations between the United States and China under Richard Nixon and Mao in 1971. Relations between China and the Soviet Union remained tense until the visit of Soviet leader Mikhail Gorbachev to Beijing in 1989.