Aluminium profile cnc build

[JAZZCNC]

#1 If I move rails to the front slot i will have bigger gap between gantry and Z axis back plate (at the moment it is 3mm) if I am using wide carriages. Maybe i could use the narrow ones the same like on Z axis?

I was going from your pics which looked like wide gap between z rear plate and gantry profile. Without seeing better view then can't comment further.

#2 How i could brace that connection? I have some of these so i could use them.
They are 40x40 and 6mm thickness.

No wouldn't use those and would go with something like your second option.