Hi, I have one outside catering company introduce to you ~ Gourmet International Catering services Ltd (web:gourmetinternational.com.hk).
This company serves a lot of famous clients with high quality food, i.e. OOCL, Philip Morris etc. and the minimum charge is HK$1,000.00. This is no worry to get not enough food. As far as I know, the cheapest menu is HK$60.00 per person (min 20 persons), of course, they can tailor-made for you as well.
Anyway, hope you can have a happy christmas party.