May I just add as well that this mod is looking really promising. It is good you are looking into the specific regiments that fought in the conflict and trying to get the right uniforms. It is a bit of a murky subject though, some regiments may be issued with one colour uniform one year, they would wear out, and a completely new uniform would be issued the next, with a mxituer of the two sometimes being quite common. Sashes are an issue as well, some Parliamentarian regiments didn't wear orange, Cromwell is seen in one painting wearing white. The Earl of Essex' army definitely wore orange though and they were the main field army Parliament had to offer.
I am sure this has probably been answered but will the Covenanter's be a separate faction? Seeing as though they fought both the King and Parliament it would make sense. And are they in grey uniforms? Most Covenanter's wore grey uniforms with blue bonnets.
With the fife and drum you might do well to get in contact with an English Civil War re-enactment regiment, they know all the necessary drum calls, and see if they can perhaps lend something to the musical side of things. I do Napoleonic re-enacting myself but some men in my regiment might know a few from the Sealed Knot.
I am sure you have probably heard this tune but I think it would be very fitting for background music:
It was composed after the First Civil War but it has been used in documentaries about the conflict. It will be good when this mod comes out, I have just finished studying the Civil War at University and have had a keen interest in it for years (my home town was the only town in East Anglia to actually side with the King, apparently you can still see a hole in the church tower where a cannonball went through).