I think it's down to personal preference really - although in my experience people tend to make the highpoly first, since it means you have a better idea of what you must do in the "final" lowpoly to get the best look. If you build the "final" lowpoly first then you should have a really good idea of how you want the final…
Yep, using a tool like Polycruncher or MeshLab (free alternative) can be very useful especially if you're dealing with a lot of zbrushed parts. I know that Epic use (or used) this workflow for a lot of their assets on Gears and Unreal Tournament - poly crunch down a zbrushed piece of geometry so it can be brought into max…