Tipping is, by definition, not compulsory anywhere ever. If it's compulsory it isn't a tip. For taxis it is positively abnormal. For restaurants you will get a 10% "service charge". You can leave a little more in cash if you are particularly pleased with the service, but it certainly isn't done as a routine thing.
HK$20 is fine for a bellboy unless you have an extraordinary amount of luggage. Even HK$10 is OK for carrying a single bag up to your room (although that might be seen as a bit cheap in, say, the Four Seasons).