Frybird said:
I actually am.
However, reconsider the whole "Author Dad who does not care" thing, because it does not sound that believable.
In fact, that is why i said "foreign girl"...being abducted and brought to another country makes it harder to find her/harder for her to get home before her pursuers catch her.
To make it even harder, you could make the "leader hobo" a paranoid wreck who refuses to get help from others (the police, people on the street) because he thinks that it is all a conspiracy with everyone working together and no one to trust
Okay lets go...
Main character
Girl works better, so I will refer to girl through this, but you could use a guy, all you have to do is change a few things as girls respond differently to guys, it will also change how the movie is seen as people are either like "Poor her, she's so nice" or "He's cool/really tough, good on him for escaping". It's if you want damsel in distress or Urban rambo/spy survival type character.
Setting:
Foreign girl is good, but avoid cliche cities. No N.Y., no chicago, set it somewhere a bit random, but you still want to keep tight packed streets, Europe is good but you want to make it somewhere with a lot of winding streets so you can get that urban feel.
Homeless people:
I like your old guy, but I say make him ex army, but a small guy/sort of squirrel like. Give him some random survival skills, a dose of crazy then let him run his team like they are in enemy territory, referring to the city, (maybe add a conspiracy plot, or just make him a bit crazy and just dislike the rest of the city) You also need a few younger mid 20 characters so you can throw in some accessible humor, and add some more to either comedy or drama depending where you go.
You could add a mother/father figure if you want to go with her parents being focused on work so they can impart life lessons etc. or you can add humor for the older crowd, or just to add numbers to the group.
After that, just a few people to round it off, though I think this idea works well if you lean towards comedy.
The parent/s:
I chose author, as it is pretty standard but anything will work as long as you make them rich, it makes kidnap more likely as they can actually pay a ransom, or depending on how serious you want this they could be two average people, and she gets kidnapped to be a sex slave sort of like in Taken. If you want to skip that you can go politician from foreign country (I'm thinking in charge of military, and the conspiracy people actually just want a military contract with her father, and use her kidnap to convince him).
The Homeless activities:
Breaking things, stealing random things, like signs from strip clubs to build their homeless home/fortress, shoes, paintball guns to engage in funny but cool city wide homeless paintball fights, anything you can think of they can steal and use in funny ways or to survive. Then maybe have them spend a lot of time doing random stuff around the city, maybe stealing the city golf from Fight Club.
Nationality:
You are going to want to make her from an country that speaks a certain language to a country with the same language otherwise you would have to figure out how to teach her a language in a short period of time.
Other ideas:
Relating to parents and conspiracy, the military idea could work but I recommend moving it around a bit, you want to change the type of kidnap to fit the story. If you want serious, make the kidnappers and situation serious, with "shut up or we'll slit your throat you *****" instead of a more comedic bungling kidnappers.
Comedy ideas
At some point have them steal mopeds.
Give them a lot of random clothing so they look quirky, describe them as wearing older clothing but you don't want them in dirty rags, just stuff that is weird/mismatched. think army surplus or just weird mash ups of clothing.
The paintball fight idea.
Serious ideas:
Main character dies/gets the homeless people killed.
Have her survive that, but be shunned by the homeless community as they hate her for getting their friends killed, maybe leading to her death as she starves to death in the streets.
Have her dad be some sort of illegal arms dealer, and have her getting kidnapped and her father paying the ransom lead to a disaster, maybe a nuke being set off in your country of choice.
I can maybe add more, I'll think about it.