Respectfully, I don’t like this idea. A shattered military unit trying to get away in small groups is one thing, but at the scale of the current armies (600 man “Divisions”, etc.) roving around with a platoon or small company represents not a shattered unit, but a sizable amount of divisional assets. Also, units of this size would face daunting logistics in terms of food and fuel, and the logistical train would be daunting and unsustainable…the old saw is amateurs discuss strategy, professional discuss logistics right? If foraging, a group this size would quickly denude the surrounding area of all food and supplies, no doubt to the alienation of the countryside. Lastly, given the size of the units described, and their potential fighting strength, there is little need to retreat, they can simply fight their way out.
In the end, of course, its your sandbox and you should do what you and your players enjoy, but I thought I’d share a contrary thought…