Tali does actually have a sidequest, of sorts. There are spoilers in this post, of course...
You can get her to discuss her pilgrimage at length and she will eventually reveal that she is the daughter of a member of the Quarrian's elected council. Once she does, she will hint that she needs to bring back something incredible for her pilgrimage and will ask that you let her know if you find anything. If you bring her along during the Geth sidequests (the one where you eliminate the four geth bases, and then follow their signal to a final outpost), you will receive an intelligence disc containing a vast amount of geth information. She will ask for it to bring back to her people -- you have the option of allowing her to make a copy or denying her.
You can also just bring her the disc if you did the mission without her in your party, but that's a bit bland.
--
And yes, the mission with the gang member only shows up if you're an earth-born. You get to talk a crazy young woman down from suicide if you were a colonist.
