RA1 seems to be a war on the scale of WW1, tactics of WWII, and Cold War era tech with some outlying weird tech. This really would be best split into a separate topic, however, as it has little to do with Delta balance.
Nah, there's no way the GDI Mammoth would have gone decades without some changes/improvements somewhere to its ammo or armor layout.
Soviet Mammoth wouldn't realistically stand a chance. It'd likely do some damage before it went down, though.