2, eat + drink, Hong Kong food is considered more expensive, usually a bowl of pieces of Meat noodle is also about 23 yuan, if by your three meals a day are eating 23 yuan. That day you need 69 dollars, 69 * 5 = 345 dollars. Drinks are also more expensive, a bottle of mineral water (small bottles) is also 5.5 yuan, this you want to think about the amount of water you drink every day.
3, line, Hong Kong is an international city, the transportation is very convenient, basically Hong Kong Island and Kowloon's transportation is very convenient, there are subway can be sat to. MTR starting price that is to say, no matter you are sitting in a station cost is 8.5 yuan, the whole about 12.5 yuan or so (specifically I have not sat in the whole, so do not know). If you're taking the bus, you'll need to bring your own change.
Hong Kong is still quite big even if you say it's not, 6 days is basically enough time for you to go through the well-known attractions, but if you want to go to Disney and Sea Sample Park, it will take more time because these two places are full of tourists. You'd better take a direct train from Guangzhou to Hong Kong will be more convenient, so that you don't need to change trains halfway to Shenzhen.