Hi Pilou

>>Seems sufficient as you can enter any number on the right's case! :)
Not any number. Constraints settings can be changed with sliders only.

>>What render do you use for your gorgeous images ?
Octane render (http://render.otoy.com/)

>>Seems sphere must be twice bigger than a "square" of the gride ?
It depends of Timestep value.


Also you can try to increase Shear value to reduce flexibility.