My take is the Army general's stack is the fire brigade. I usually make it the weakest stack, but just by a bit, say 10 points, never weaker than 100 points of the smallest corps. For some reason, if it is not, in fact, the strongest stack, but never-the-less a strong stack, it seems to always join the battle very quickly if one of its corps in an adjacent region is attacked. I do find making the army the strongest stack in attack, using parallel marching orders, usually brings all elements of the army into battle the most quickly. So a bit of a re-balancing as initial shifts.
As a bit of a side note, players who are studying how MTSG works should use Rise of Prussia. While it works pretty much the same in all games where it is available, the complexity of challenges Frederick faces can only be met by mastering MTSG. So playing as Prussia players will be challenged to learn corps/army relationships.