@Frank_G:
Yeah you are right !
I didn't take account the time production side
If you want to go from a base mesh and just vary surface to make different characters : that's another solution.

Indeed when retopology you must keep a good polygons flow.
It's lot more easier to make low polygon by retopo than from scratch.

For Zbrush i found that :
decimation master plugin

It can be a solution to bring down the very high model or micro detail mesh to a high detail that can be loaded on your modeler where you will retopo.
For retopo you don't need at all the very detailled model !

Another soluton used by this great artist
Bishop tutorial
On Zbrush you can subdivide you model, and you can at any moment subdivide back.
So he just subdivide back the mesh to a lower resolution to be able to import it on a modeler.

Ok i stop for now
It was just me seraching how could i do retopo easily from Zbrush models (i think i'll do like iin Bishop tutorial).