i generally make a highpoly first.. its a pain in ass to make a lo first and its just a horrible workflow.. you can make a model that is low in polies for zbrushing.. just use all quads.. zbrush hates triangles.. even though there is a convert tris to quad button in zbrush i just never used it.. so i dont know how that…
You can zbrush onto the low poly then bring both meshes back into max for rendering normal-maps, or you can generate the normal maps in zbrush but i would not recommend doing the latter.