Balancing lists

I don't believe there are any guidelines for building your own lists.

From what I can gather from other examples the goal seems to be about 8-12 units with a total VD of about 20?

they have broadly the size of BI2 armies, but with less limitations. As this is not a tournament set and as orders of battle may differ widely from battle to battle (at least for some periods covered by the expansion) I would work on balancng historical opponents without care of these army can perform with others.

Veterans (and elite) are pretty strong, so don't exagerate. Also musket A or machine guns are strong, so use with care.
One thing that can help the balance is to give one or more rerolls to the side you feel a bit weaker.

Another example is if you feel an army should have horse artillery instead of foot artillery, give the them the horse and balance this bonus with a initiative reroll for the opponent.