Regarding the Prussians:
In 6mm the differences such as they were will not be visible (in 15mm they would).
Infantry/Dragoons/Artillery;
The Pickelhaube and Waffenrock tunic became the standard Prussian uniform in 1842. There were minor changes to the tailoring and detail of the tunic through the period but by and large it was the same.
The Pickelhaube helmet started out with a taller crown that was lowered to the more familiar profile in the 1850s.
You will need to be careful with the non "Prussian" elements of the army as Baden, Bavaria, Brunswick and Westfalia adopted different aspects of the uniform at different times up to 1866 and even after retained some unique colour/style differences.
Hussars;
No real changes after introduction of the tail-less tunic and Busby in 1850
Uhlans;
No changes after 1840 introduction of the double breasted tunic and smaller czapka.
Kurassier;
No changes after introduction of the tail-less tunic and Polished metal helmet in 1843
Excellent guide here:
http://www.kaisersbunker.com/gtp/index.htmRegret I can't help with the other armies you listed....I game 1870 FPW Corps level (with 6mm Heroics/Ros figures and 2/3 scale Volley & Bayonet rules).