My impression is most everything in the slums his hijacked from the plate to some extent. Little or nothing is obviously planned by ShinRa, so providing resources is likely not a thing either. One look at the teaser trailer's example of the massive number of electric wires everywhere kind of enforces this idea. Either that or there's likely "Middlemen" that offer lower quality mako energy at a much higher price, and everyone basically steals from everyone else to some extent.
Water in at least part of the slums seems to be sourced from that waterfall around Aerith's house, as you can see pipes jutting out from the foundational earth under her house.
So one of the questions I really want to ask, if there are any architecture and/or physics nerds is: How much weight is an entire city, and what would you need to support that? How do you keep it all from just collapsing, if at all? Or is this going to be mythbusted as something only possible in fantasy?