How large does the European population have to be? It could be feasible for some European power to make more of an effort to colonize Taiwan than the Dutch did, overwhelm the relatively small native population with settlers, then have China conquer the island later. Provided Taiwan stays relatively isolated for as long as it did OTL, you could have a substantial population on the island with mainly European ancestry. They would be a fraction of a percentage of the total population of China, but then so are the vast majority of China's ethnic groups. The trick would be coming up with an incentive for settling the island so far from Europe with relatively little to offer compared to the Americas.