Here's my cents. What's ethical and what's legal are 2 separate things. Also, photo essays on homeless have been done before. What can you offer that is different and helpful as a photographer? As a photographer, are willing to go beyond what's on the surface? Gordon Parks photographed impoverished in the favelas of Rio. He lived with them for a while to gain their trust. How deep are you willing to go?
Exactly. To me it has to be about an honest end result, with a desired effect to accomplish something that is full of emotion, compassion, and somehow of benefit to all involved (ideallymostly of benefit to those that are homeless).
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh