You could even add the Warlord plastic Landwerh Prussians, same uniform no change, Perrys BIF Guards for Danish and Perrys Carlist wars Isbellino have some bell tops that are suitable, Northstar 1866 range can be used for the Holstein army, just paint a white arm band on as this was the same uniform as the Prussians.
Any ACW with frock coats would do for the Danish, i use Redoubt and i got plenty of spare heads which will do for my ACW plastic artillery guns.
I would say Foundry seem to be abit on the small side compared to the Northstar range and Redoubt, love to get hold of the Eureka miniatures but its almost £2 a figure and the amount we use is big battalions,then you can use some Americans from the Mexican war range, this was the old uniform of the Danish army and the Holstein wore them so not to confuse them as Danes.
I think also Old Glory do some nice looking Prussian Horse Landwerh which can be used for Holstein horse, any ACW on horse with frock coat can be used for the Jutland sharpshooter Corps, and last any civilian in the 19th century, just get the green stuff and create a sash paint it in the germany colours of black,red and gold and hey presto you got a Volunteer Holstein unit.