My first 700 x 500 x 300 Mill/Router machine

[Lovre]

Think you will find that 10mm aluminium on its own is nowhere near ridged enough for the gantry especially if you want to cut aluminium. Plus how do you intend fitting the top supported rail if gantry is only 100 wide or have I picked you up incorrectly which is more than likely.
You also seem to have quite a large z axis travel, what's the travel on the z axis??

I just found shop that sells 15 x 120 . Will this be sufficient ?

I was thinking of combining, maybe putting vertical parts of Z axis in 15 x 120, and rest of horizontal parts in 10 x 100.

I'm a little worried about weight of whole thing(15 x 150 is 6,2 kg/m, and 15 x 120 is around 5kg/m), so this is why I want to combine different thicknesses to save some weight.

Travel is not so big in my opinion, it is some 250 mm (rails are 350 in total).

Horizontal parts don't seem to be problem cause I think SBR16 is going to make whole construction tougher.

I'm actually thinking what could I use on side of bottom plate, maybe 50x50x4 mm aluminum tube ?