My grandfather was a former member of the Chinese Nationalist armed forces. He used to talk about how terrible life was for both the Communist and Nationalist troops. One thing he mentioned about the Chinese Communist troops that intervened in the Korean War was that a significant portion of them were ex-KMT troops that had basically been forced into the PVA. He said that it was his impression that Mao wanted to get rid of these troops of uncertain loyalty in case of a US-backed KMT counterattack, and had been given the chance to do so in Korea, where many of these ex-KMT troops would be sent in first while more loyal, elite, Communist formations would be committed after saturating the UN positions.
I used to think there was at least some exaggeration in this, but I recently read that one reason why the ceasefire in Korea took a long time to negotiate was due to a significant amount of Chinese POWs wanting to be repatriated to Taiwan, not China. Do we have any idea as to how many ex-KMT troops were involved, and whether there was any truth to what my grandfather said?
No doubt many KMT troops surrendered and many joined the PLA during the Chinese Civil War. (Around 1 million or so surrendered).
But the PLA itself was mostly composed of ex-KMT military, and it was already at 1 million troops before it engaged the KMT military in the Chinese Civil War. If you want to get technical, many CCP members were once KMT members. They were trained in the same military schools. Mao, Zhou Enlai, and Chiang Kai-shek all knew each other very well.
But as far as the Korean War, I don't think the PVA was composed of large number of ex-KMT troops.
AS far as the PVA troops who wanted to be repatriated to Taiwan, that was true to a point. It was well known that US brought in ROC interrogators to Korea to perform propaganda "brainwashing" (as much as US accused PRC of doing to US troops).
At the end of civil war, most PLA formations contained large number of former KMT soldiers, for reportedly 1.8 million KMT soldiers switched side and joined communist from 1945 to 1949. But if these ex-KMT are mixed in existing communist formations, obviously you cannot selectively send them to the front first.
If you look at the Korean war order of battle, ex-KMT formation deployed as a whole at the beginning of the war was the 50th Army (former KMT 60A at Changchun, surrendered in early 1949), but this is just about 1/10 of all PVA formations. So even though there is some truth in what your grandfather said, it is no doubt exaggerated.
BTW the 50A suffered great loss in PVA's 4th offensive in early 1951, so it had to be pulled back to China to regroup, later it was sent in again but mainly for coastal defense in North Korea. The 50A was eventually disbanded in 1985, only one division remains.
Source: 中国人民解放军各师沿革, 中国人民解放军战史.