I'll go for the top one so, I wasn't thinking that it could have any bending problem, thanks.

Limiting factor is the gauge of the extrusions on the bed sides.
I'm not sure to understand it. Do you mean the extrusion is it too high?

Good idea for the end plat, do you think 5mm aluminium is enough to do it?

I found this documentation about hiwin rails : http://www.hiwin.com/pdf/linear_guideways.pdf
It can be interesting for someone. Is it a good choice to have a HGW (wide flange carriage) could be a good choice for the Y axis for example? And HGH for X and Z axis.

Any advice higly appreciated.