Thanks Paul for your replies.

Plenty of great options in the instancing process. I found the octane process sometimes tedious. How work the density map? can I drive the instancing to put more grass near a wall for exemple (I like Vue ecosystems!)?

Too bad for the manuel. I like to read them in the train! and yes I already viewed all videos (and not only once!) 

I choose to split my scene taking in account the shadows to optimize the postwork. It's not easy and some object can be in both render. Aerysoul clothings (top quality!)  have an impressive number of textures. 

The time to create a new material preset is very long. Collect all textures and resize them all is quick but after I must assign all materials to resized textures. Another inconvenient is that, for me, the ratio is not the same for each scene: it depends of the complexity of the scene and if the character is in front or in the background,....  Do you know some tool(s) to optimize the process in Poser?