Chinese-friendly Kazakhstan existed as a buffer against the Ottoman Empire originally. Though with their fall Chinese military interest shifted to keeping a watch on the broken states to the north in an attempt to forestall its spread to the former Imperial territory in the south, or to provide an extra spring-board location should they need to flank.

Persia, as rising a power it is, doesn't really have the capabilities to fight China and I remember Veo saying in IC at least once that if they entered a protracted conflict with China they'd suffer economically. Though Chinese troops in the region could also be used to prevent the growth of Perisa to prevent it from encompassing Greater Iran.