Soldier for soldier it's optimum to have 1 gigantic stack and multiple fortified tiny stacks for each unit type, yes. Unfortunately you can only put your own units in, and usually as general you'd have some of the large stacks, meaning you'd need to get lucky and have it attack your fortified stacks, and only applies to unit types that have multiple different units in that point of the tech tree. (Horsemen/mobile are pretty cool for this except for that awkward tank-only era)
What I'm saying is you'd still want as many people to be online to replace their own sponge stacks. The general alone is invaluable in being able to shuffle orders around depending on how the battle goes but alone he can only go so far.