Actually, walker bots would be perfectly fine in the rugged terrain on mountains, where they'd have a massive advantage over wheeled and tracked vehicles. Although they'd probably be neither giant nor bipedal: just swarms of small mechspider tanks.
I don't know about this, due to the square cube law, the foot print to weight is going to increase exponentially faster than it's area. Walking around a mountain where loose ground, high inclines, and generally unstable ground incapable of supporting a dozen tonne footstep doesn't seem like a good idea to send an incredibly expensive and delicate piece of machinery.
However, the laws of physics say that recoil on a railgun doesn't matter to ground-based firing platforms (from human rifleman to megamecha) thanks to friction.
This is mostly true, but the main issue with recoil for large caliber guns is balance and generally the robustness of the chassis to take the force without messing up the precision instruments. For tanks and other armored vehicles with large armaments, as far as I know the guns are always placed in the center so shooting the cannon doesn't displace the aim of the gun. I think the aluminum tanks used by the US in Vietnam encountered this problem when they stuffed missiles into the turret.
Does anyone know if spider like legs scale up? From the way they're attached to the bottom of the body to the shape and contact with the ground, would the square cube law mess with this design? I don't see this sort of design being around past the tarantula size, and there's probably some sort of reason for this. Is it due to the increased mass of the leg relevant to the area and the strength of the materials for the joints involved past a certain point in size?
Most large animals seem to go for the straight legs directly under the main body like elephants and rhinoceroses.