Holly, these are all good questions, and ones that I asked when I first got into photography and the Forum.
First an anecdote. The DC Meetup #3 (I think--the one with me, Kelox, and JonMikal) gave me a great story to tell about people looking at you funny. We were in the Kennedy Center for the Performing Arts, and we were up in the KC Cafe. I was busy taking pictures of the salad bar and didn't notice the manager coming up to me. He said, "Can I help you with something?" I just smiled and said, "No, I'm good. Thanks." and went on taking pictures. Later than night we got a great pic of JM looking up Marylin Monroe's dress. The guests in the hotel thought this was quite funny, but the management didn't. Fortunately we left before they could say anything.
The general rule (law) is that anything in public spaces are fair game unless you are planning to make money off it...in that case you need consent forms and all that.
To be honest, I wouldn't have been so brave with that manager if it weren't for the fact that I was with others...I probably wouldn't have taken the pics if I was alone.
As far as homeless, or any people in the street, I've realized that there are several ways and styles. Some will pay the person (or give them some money--however you want to look at it), some will just take the pic and not care--again remember that if they are in a public space they don't have any real control. Some shoot from a distance so the person doesn't even know.
I struggled with this a lot, and I realized a few things. First, most people don't care if they get their pictures taken or not. Most peolpe don't even realize that your taking their picture. I still feel uncomfortable, so I understand and don't really take that many pics of strangers.
I think that there is also the reason you are doing it to consider. I personally love Mansi's work. I'm fascinated by other countries and other cultures. I think that Mansi has a way of showing the people, showing who they are, showing their life and what's going on (there are others on the Forum that are good at this as well, but I thought I'd make Mansi uncomfortable

). I don't think that she ever looks through her camera at the people and says, "this will make a great pic to exploit this homeless person." Rather, I think that there is real empathy in her shots, and that makes the picture have a very different feel. I don't feel it's exploitation, but rather trying to teach me about this person and his or her plight (this is not meant to imply that you think she is exploiting anyone...just trying to make an example).
So, ultimately, my long rambling post is really just trying to say that if you really feel that you need to take those kinds of pictures, there are ways to do it so that you don't have that guilt...why do you want to take the picture? Do you just think the guy laying in the gutter is funny? Do you think the picture will show how much better off you are than they are? I think the answer, of course, is no...you sound very much like you feel for these people and if that is what you want your pictures to evoke in us the viewer, then always have that in your mind when you shoot...always say to yourself that you are telling the story of the less fortunate in our society..that you want us to see that these people are human, they are someone's brother or sister or son or daughter.
On the other hand, if you can't do it, don't. There are so many other things to take pictures of. Also, don't force or rush it...if that is really what you're drawn to, you will get more comfortable with time.
Hope that helps...