Well, if you want to let them tell their story, that's great! But perhaps keep the cash incentive out of it. Perhaps offer something else, like a meal or a cup of coffee or something tangible they can use. Also, establish a really really interactive informed consent policy. Make sure they really understand what you're going to do with their information and story. A standard form probably won't be good enough, since you have to be particularly cautious when it comes to potential for coercion.

The problem with the Ted Williams story is that although he got the big breaks, the story didn't stop with the happy ending, and his life spiralled back down again. He went into rehab but checked himself out after only 12 days. I'm not saying that homeless people are not able to be helped, but that it takes a much more structured support system than a video going "viral", and there are lots of good people working to help homeless people back into work, building up their skills, etc. I'd recommend, if you still want to concentrate on homeless people, to go volunteer/work with a charitable organisation for a while and get out amongst the people so you can get a bit more of a perspective on things.

To be honest though, I think the more general approach to interviews is a much better idea.

I'm always happy to chat about ethics, since it's something I am quite passionate about (especially research ethics, vulnerable populations, and informed consent - so you really hit all the targets for me with this project! heh) :)