A good place to ask questions like this is one of the Expat forums for Hong Kong such as http://www.geoexpat.com

The short answer is no you don't need heavy duty raingear. The rainy season extends to about late September / early October. Normal trainers/sneakers are fine for walking around.

All the climatological data you could want here: http://www.hko.gov.hk/cis/climat_e.htm