in reference to the Soviet Invasion of Xinjiang (1934) and the Xinjiang War (1937).
I can't find a satisfactory explanation. The white russians were anti-communist, and I thought they would've been outlawed by this time. why would they assist the soviets, why would the soviets allow it, and why for the defense of a pro-soviet, quasi-communist regime in Sinkiang?
Because by this point the label 'White Russian' was just another way of saying 'Russian émigré'. The White movement, such as it was, had long since dissipated, leaving tens of thousands of Russian refugees in China to fend for themselves. Plenty picked up peaceful occupations or returned to agriculture but the chaos of northern China during the 1920s and 1930s also provided opportunities for former White soldiers.
These soldiers slotted in well to the complex environment of the borderlands, which was something of a Wild West at the time - a weak Chinese government overseeing a mix of local warlords, organised bandits and foreign agents. Scattered Russian military detachments found ready employment serving under Chinese warlords (eg Zhang Zongchang) or as bandits and mercenaries. But they did so on their own initiative: anything resembling a unified White movement (arguably always a fantasy) had long since disintegrated.
These White soldiers would have been generally anti-Soviet - this feeling was certainly reciprocated, with eradicating White settlements being one aim of the 1927 Soviet intervention in Manchuria - but they weren't ideologues. They couldn't afford to be. Whatever about the occasional raids, everyone had to accept that the USSR was going nowhere soon.
In this particular case, the White Russians were just one part of the combustible mix of populations in Xinjiang. And this wasn't just soldiers - there was a small permanent population of Russians in Ürümqi, which was an important player in the fairly impenetrable ethnic politics of the region. In the early 1930s these Russians were aligned with a series of Chinese warlords, forming a core of Sheng Shicai's army.
When the latter requested Soviet aid to quell unrest in the 1930s, it led to the ironic sight of Red and White soldiers working together. But while it was surely odd to have White soldiers serving under Red officers, both sides had a stake in quelling the Muslim disturbances. The memories of a decade-old civil war were far less important than the concerns of the day.