I absolutely would have them of different classes. Tandy being more well to do makes a lot more sense. And it's not that unusual either - Heroin use for middle class white people is at an all time high. It's very easy that she could be on the street doing drugs. I'd actually play against stereotype and essentially have Tyrone rescue her (it's just people from the outside won't see it that way). I don't know about framing them for a crime, the original story works just fine - they're runaways on the street, they get experimented on by bad people, they get powers, the bad people still want to get them.
One thing I remember about the characters (this may have been the 1610 version, my introduction to them in 616 was shortly before so things tend to blend together) is Tyrone's insecurities that Tandy is too good for him. A lot of it is his projecting the views of society onto himself. Basically, teenage angst is good, just make it one with a purpose.
