As a foreigner who also speaks Mandarin and has been in Hong Kong plenty of times, even lived there for 7 months, I can say that even though many people claim that Hong Kong people mostly speak English, in reality it really is not like that. Especially people in stores and restaurants do not speak any English at all most of the time. Others, young people, might speak English but are too nervous to speak it or had little practice so they will not understand you very well. I think it is always good to learn a new language, especially if you go to live in their city it shows you respect them and their local culture. Cantonese is a great and awesome language. If you speak Mandarin, Cantonese won't be too hard to learn. You just need to get used to the tones a bit, but furthermore it is at least 80% similar to Mandarin.

香港人的英語還好, 走在街上都是中英雙語標誌, 因此也沒有大問題, 不過如果你懂得粵語, 你在香港生活會比較像本地人, 也比較好玩... 香港人英文都 ok, 行晌街你見親都係中英文標誌, 應該唔會有大問題, 不過如果你識得廣東話, 你晌香港生活會 local d 同好玩 d lor... Don't worry about the English used in Hongkong, some ppl are very good in it and some not. If you speak Cantonese, you will experience a more colourful local life in places like Mongkok. Considering you speak Mandarin already, it shouldn't be very difficult for you to learn Cantonese (think Portuguese <- Cantonese VS Spanish <- Mandarin). Good luck!

Able to learn more languages is always better. Cantonese is more colorful and earthy compared to Mandarin and could be more difficult because there are more than 8 tones. However, here's a link to Carlos Douh. As you can see he speaks like a true native! https://www.youtube.com/watch?v=geuqYUVerQ0 Generally you can get by HK with English. Most people will understand conversational English. However, how good their English is, depends on their educational background. All road signs have the English equivalent and so do most shop signage.