Units can be organized into custom formations, however this doesn't always work. Especially with artillary the unit order is often changed:
Again, in most cases I can form a custom order and send units into battle respecting that order. This problem seems to occur when artillary is mixed with infantry. Here's a screenshot of this happening with a group of two bolt-throwers, White Lions of Chrace and Lothern Sea Guards. My custom order is:
Sea - Lion - Bolt - Bolt - Lion - Sea
The order when I click and move the units around is:
Bolt - Bolt - Sea - Lion - Lion - Sea
So it seems the artillary is pulled out, while otherwise the order is respected.