Travelling to Hongkong during october

The average temperature is about 25 C at that time. It is still during the rainy season, so occasional squally thunderstorms may happen. You'd better pay much attention to the weather forecast as you are outing. Short-sleeved shirt is OK then, but you can also take along the long-sleeved shirt in case of the heavy, sudden rain.
For the summer-like clothing, you may find them in some shopping malls to give discount for quick turnover.