One more thing: let me be the most cynical I can for a moment: not all homeless people are sage-like gentle folk who've had nothing but bad luck and need to be treated gently and sensitively. I am NOT about to get into a conversation about how someone ended up homeless, I will NOT blame the victim, and I KNOW for SURE that I don't envy their life. Having said that, I know guys (me included at college in NY 20 years ago) that would ask these guys to take their pictures in return for $5 or a six-pack of beer. A lot of these guys don't want to be treated like charity cases that need to be cradled in strong defending arms, they would rather be dealt with like grown-ups, or better even would prefer it if you just gave them a bottle of something decent, take your pictures, and then go away. If you REALLY want to take expressive pictures of poor people, think about compensating them for their modeling work. And if they prefer booze over money than don't be moralistic or condescending - either give them what they want or go find a different model.
...and have some sense about you. Some WILL beat you and sell your camera.