actually, I've found a way to get them all to fire at once. Delete the "Fire_By_Rank" attribute, the "Start_skrimish_on" attribute and the "gunmen" attribute in the export_dcr unit file, and launch the game with the "my mod" thingy (look how in mod forum).

I figured this out after I got kingdoms, and lithuanian arquebusiers all fire at once. Checked unit stat in the unit file and found those attributes missing. Deleted them on some gunpowder units in org m2tw and it worked like a charm.

Enjoy!
Per Ole