Sorry for the late reply! Just got around to regularly Redditing again.

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So, this question is a bit complex, but is helped along by the fact that both formal and informal relations between these two entities was quite brief. In fact, it lasts only about 27 or so years, from Sun's possible (though not confirmed) meeting with Soviet agent Grigori Voitinsky in Shanghai, 1920 to the collapse of the KMT in 1947-49. I will divide this into two sections, 1920-1927, and then 1928-1949.

1920-1927

While I don't need to tell you the foundation of the USSR in 1917 was extremely unique and changed world history considerably, it should be noted for our situation that the very principle of the USSR was "good" for Chinese nationalists. Not only was there a new, anti-Western power in the East, but it neighbored and took considerable interest in China. As early as 1920, while still fighting the civil war, we do know that Soviet agents reached Beijing. Despite the fall of the Empire of Russia, there was still some controversial issues that needed to be solved between any new Russian state and the Chinese. Of course, this was the issue. In 1920, both what was the legitimate "Russia/USSR/whatever" and who exactly was in control of "China" wasn't a straightforward answer. There were still Tzar loyalists roaming around Asia, and there were about four or so cliques of warlords claiming to either be "China," or independent of China.

The particular issue affecting Sino-Russian diplomacy was the Chinese Eastern Railway (CER), and previous Russian extra-territoriality. The first issue we will come to later since there will be a transfer of power several times in Manchuria from 1917-1931, but the second one was a particularly sticky issue. How can a state claim to be the defender of individuality, of sovereignty, for all nations and ethnic groups, if they have extraterritorial status in a foreign land? The issue was never really resolved when the Tsarist regime came to an end. But again there's another issue. Some warlords, particularly Duan Qirui and Wu Peifu, who are at their height in 1920, don't really care about the issue. In fact Duan's taking of a Japanese loan in 1916 to "prepare China for entering WW1 against Germany" was entirely spent on himself, while he gave the Japanese access to railways in Shandong province. So when the Soviet agents entered Beijing to talk business, they kind of found themselves turned away, or at least ignored. Agents found much better success and amiability from the newly found KMT (GMD) and Sun Yat-sen.

By 1920, a year after the foundation of the KMT, Sun and the gang were really no more powerful than they had been when Song Jiaoren was assassinated in 1913 by Yuan Shikai. Actually, due to the National Assembly being dissolved by Zhang Xun in 1917 the party held zero political power in any part of China- except Canton/Guangzhou. There, Sun's son, Sun Ke, was holding things down as mayor of the city, but was under constant harassment by forces loyal to local warlord Lu Rongting, who wasn't too interested in any sort of unified Chinese state. Sun had tried again and again to rally the support of America, UK, or France, but through all this he only had minimal support from huaren, overseas Chinese populations who funded his adventures. The USSR decided to dispatch Adolph Joffe to keep direct dispatch with Sun and the party, most importantly his goal seems to have been twofold. On the one hand, the USSR was to secure an alliance with, and then help aid in conquest, the KMT. The humiliation of the Russo-Japanese War was still freshly felt on the faces of Russian revolutionaries and nationalists, and China would be as good an ally as any against Japan. the Soviet's control over Siberia and the rest of Eastern Russia was still tenuous at this point, so a large and strong ally was deemed essential. Secondly, and interestingly, Joffe's mission also seemed to have been to force Sun and the party by extension to accept a form of communism as its doctrine. While Sun was willing to work with the CCP, he never did fully trust them; in 1921 when Sun came into contact with the famous Dutch communist Maring while he was in China, Sun reportedly refused to accept Maring's suggestion that the KMT should reorganize to include both peasants and workers and establish a "revolutionary army" as well as cooperation with the CCP. Still, Sun needed allies and aid, fast.

Joffe was able to secure whats known as the Sun-Joffe Declaration in 1923. While not "official," since the KMT wasn't the recognized legitimate party of China yet, it allied the two entities with two reservations. First, the Soviet system was not to be accepted in China. Second, Joffe reaffirmed the 1919 Karakhan declaration renouncing all the privileges that Tzarist Russia had within China. Interestingly, there is no mention on paper of Soviet aid to the KMT, just an alliance. Despite this, in March 1923 the USSR sent military and political advisors to Canton as well as USD 2 million in funds. This deal was important in isolating other claimants to the KMT, like Chen Jiongming who had betrayed Sun years earlier in Guangdong. Now, for the first time, Sun was the indisputable ruler of the KMT, and had actually brought something monumental to the party; foreign materiel aid. It should be noted though, that these meetings between Sun and Joffe weren't without quarrel. Sun was well aware of the rumors that other Soviet agents had made agreements with the Zhili Clique to ally and invade Manchuria in order to secure joint Russian ownership of the CER (which we'll get to shortly!).

In 1925, Sun died in Beijing, leaving a power gap in the KMT. While Chiang was his preferred second hand man, there wasn't really any written will or rule on how to address succession in the KMT. After a brief struggle, Chiang, who was head of the Whampoa academy, came out on top. But the KMT still only had control of the city of Guangzhou, and a very outlying regions in Guangdong. The CCP had been hard at work organizing peasants in the countryside, but they had also been causing issues back in Guangzhou. On March 20, 1926, a KMT navy ship, the Zhongshan began acting fishy, with the communist captain generally disobeying Chiang's orders and lying about other orders. When it became apparent that this was most likely a move to dislodge Chiang from power, Chiang declared martial law and led loyal troops into the city. Various CCP agents were killed or imprisoned, but because the Soviet's were implicated in aiding the CCP into the attempted coup, Chiang took the step to arrest the top two Soviet advisors and send them back to the USSR. He then made an agreement with Stalin that Soviet advisors could stay, but they would have to be vetted and hand picked by the KMT, which Stalin had no qualms with. Indeed, Stalin would cement relations with Chiang and push aside the CCP for a while after the Northern Expedition, which was launched that same year.

With Soviet aid and CCP help, the KMT established control over most of China by the summer of 1926, stretching from Guangzhou to Beijing. Chiang, now in control and at one of his several political zeniths, took the step to purge the CCP elements from the KMT starting in Shanghai, and exile all Soviet advisors. Chiang it seems wasn't fooled by all the USSR had to offer. In fact, the conquest of central and northern provinces in China put into the hands of Chiang certain correspondence between other Soviets and the Beijing warlords, in which the USSR was hoping to ally with them in exchange for extraterritorial rights and of course control of the ever-so contested CER in Manchuria. Soviet support was a double edged sword that Chiang would have to come to rely on throughout his entire mainland regime.