You know the wire-reinforced plastic strips used to secure plastic bags with twisted ends like the ones sliced bread is sold in? Just used four of these in a project.
Seems fine in a pinch. Everything is so low (voltage and current both) that I don't think it matters too much. Plus breadboards imply temporary tests. If you're doing something more permanent like a breadboard CPU like Ben Eater then probably not.
Yeah, that ought to be fine. Breadboards aren't rated for fast signals or large currents. So not really a concern.
Network cable is a good cheap source of jumper wires. You can also save the bits of tin that you clip off various components -- over time you'll end up with way more than you need. These are easier to bend into shape.