tadk, let me make arguments for the two classes you left out:
Barbarian: Imagine the vast dumping grounds on the edge of the city, clustered around it and in it are hovels made of junk (for a real world example see Mexico City and others.) These are the poorest of the poor, no education, no money, no hope. Their only means of survival: gleaning off of the waste of the city. The dump covers several square miles, filled with mutant animals and dumped experiments that have taken on a life of thier own. A few of the denizens of this area have developed their own, ritualistic culture, at the center are the warriors who channel their anger and frustration with their existance into a battle rage that lays waste to their enemies.
Urban Barbarians, no mechanical changes needed.
Sorcerer: The wild manna that produces mannamutants sometimes unlocks a primordial connection with the manna field in certain individuals. These become Sorcerers.
I would probablly remove the familiar(or allow a substituion), grant Eschew Materials, and maybe require at least a 0 Mutant Point Cosmetic mutation.Gun Mages could be handled by a series of feats available to Sorcerers only.
Oh, and I think a setting like this should have lots of different ways for magic to work, thus allow Clerics, Wu Jen, Warlocks, and Warmages (and I am forgetting one from the Complete books but no matter). A vast city would have lots of ethnic neighborhoods, which might bring their own magical traditions with them.
I'd also use the Bard from Complete Book of Eldritch Might, for a totally different kind of magic.