This is how I'm doing my 6mm horse and musket units at present.
I started of with one 60mmx30mm element per battalion for Polemos:GNW but found that I can't get on with the rules. I'm now planning on using either Black Powder or Maurice. The pseudo-four-element approach should work for both. It also saves two strips of figures per 120mm wide unit!
I've yet to attach flags but the front rank unit is the Semionovski Guards whilst the second is Sweden's Regiment.