I personally do not see anything wrong with photographing homeless people. I’m capturing the street scene and they are simply a part of it.
Some of my street shots with homeless people are currently at the Pearse St. library as a part of an exhibition.
Although this is an old post it really is worth while looking at istockphoto. It takes a while to build up a good selling portfolio but when you get to around 1000 files the money becomes pretty good as well as giving you a shot at contributing to getty.
Just keep in mind that not everything sells!
I am selling stuff on 8 stock sites and I have to agree that istock is one of the best.