Here are some hard facts of life:
1) You have a legal right to shoot whatever you want in a public place.
2) People have no legal right to grab you and beat you up, or smash your camera if you take pictures that they do not want taken, or they think that you took pictures that they did not want taken, but they still have a choice to do so.
3) In a court of law you would probably come up tops, but some physical injuries can last a long time, some will never go away.
So you can go, fulfill your interest, live your dreams, just keep in mind that there are loads of people there who might not want their photograph taken, e.g. people who do not want their wives to find out they are there, people who do not want their friends to find out what they work as, people who do not want to be seen in such a place. In short, my sense is that in Geylang, you would find a lot more people willing to ignore the fact that they have no legal right to do things to you.
Cheers.