The population would be similar to the Philippines which also had a substantial European population move there, but not enough to overwhelm the natives. There would be substantial race mixing with different gradiations within the populace. There would probably be something similar in Mexico and Latin America where the upper elite is very "white" with a lot of European features, a large mestizo (Eurasian) population, and an even larger indigenous population that preserves most of the original appearance. However, everyone would speak the European language and follow the colonizer's form of Christianity.
Taiwan was always peripheral to China or the other major powers, so it won't have much impact on history. The first major change is that when Japan modernizes, it likely won't conquer and take over Taiwan as they did in the First Sino-Japanese War. If we ignore all sorts of butterflies, it would be interesting in the case of a Spanish Taiwan whether it would become part of the USA's Far East colonies along with the Philippines. In any event, it won't start impacting large scale history until the late 19th or 20th centuries.