Currently, I have a little over 80 dwarves. Half the time, 70-80% of them are idle; the other half I've given various hauling orders and have no idlers. I don't worry about idlers much, for reasons already stated.
really? Cause Dwarf therapist (the liar?) says something about "-1" for not having a job. I've been micro-managing my forts like a crazy dum dum to make sure everybody is working, all the time.
My dorfs also tend to get stuck on the roof and die of thirst while building floors. and for some reason I cannot build walls on top of floors. How am I supposed to make several levels?
I'm pretty sure dwarves don't mind being idle. They usually enjoy work, giving them one happy though, but I've never seen one complain about being idle. Not sure what the -1 is for; I've noticed it too.
Don't build walls on top of floors; build walls on top of walls. Walls have a floor above them - their top is a floor. You
can build floors on top of walls, but most of the time you won't want to. Constructed floors block walls from being constructed. Walls can be built over open space, as long as they're connected to something, so there'd never be a need to have a floor down first.
If your dwarves are getting stuck on the roof, either build scaffolding or organize better building orders: First In Last Out, and all that.