I don't see why it should be optional in the start game menu if it can be easily ignored even if implemented to all.
It can be implemented in a very simple way:
You have 10 slots to place units.
First 5 are front line, the others are in the back.
The only thing left is the ability to switch the order of the units in the army and to have an option for a blank space.
Don't want to use the ability? just use the default and don't bother changing the unit order, that way units enter the army in the first open place (more or less the way things are done now).