Do you not play currently? You have to pick a new team each time the scenario changes you aren't automatically granted a spot on your prior team.
Team stacking is not a radical concept. The scenarios should have team limits, regiments, and even sides if possible (replayability > pure history worship) attached to some script file that can be modified on the server-side for events such as generic unit vs public.
The more I learn about the Civil War the more I find just how hodge-podge it was with constant skirmishing action and small-unit initiative taken. It's often represented cleanly but it's usually not. In the most easily-known example nobody is currently claiming that the elaborate maneuver by Chamberlain happened the text-book way it was once thought on Little Round Top. Everyone's saying it happened more impromptu as was the Alabama assault tactics. I was there on the ranger program at the spot yesterday afternoon. It's what the park rangers and the guides all subscribe to anymore since as long as I've been going. Few resemblance of a functioning line on many parts of the battlefield (certainly not standing for a team morale bonus) in that type of terrain. But don't tell that to cinematic heroes in the servers.