Yes, you need some open slots for the supply. Granted, if you create the scenario with no detachment capabilities you can probably create a pretty big battle, but the ability to detach units and task organize is one of DA's greatest strengths. If you take away the abilty to do this, then my interest in a scenario decreases significantly.
This is the sticking point with DA and one of the primary reasons the sim just never found any real fanbase with wargamers. It was born as a training tool for the schoolhouse, and just has too many limitations hardcoded in. Don't get me wrong, I like DA because it is a fascinating application from a number of angles and can be used to fight out some great battles. But at the end of the day, most wargamers are going to find the restrictions and abstractions a little bit too confining.
It's a shame because I enjoy what DA does have to offer. With a rework of the database, and some significant enhancements to the GUI, DA could find a solid fanbase. But that isn't going to happen in its current form.