For effectivity you shouldn't create stacks out of both soft and hard targets ... also speed shouldn't vary that much ....
When I have the troops I organise my armies the following way:
Theatre (Europe, RUssia, Africa, etc.) (1x3 divs as Reserve)
- each with x Armygroups (1x3 divs as reserve)
--- each with 2-5 Armies (1 Field Marshal)
----- each with 4 Armycorps (1 FM (see above), 1 General, 2 LGs )
------- each with 2-3 divisions of the same type, i.e. inf, cav, mot (with the occasional brigade thrown in)
but I do mix several types of Arm and Mech, depending on their speed
Likewise as Germany I use stacks of three in my Navy an Airforce
Paras in groops of 1 or 2
Garrisons 1-2 inf (usually one AT-brigade) + 1-2 Militia
I'm against mixing slow (6 kph) and fast (10 and above) units as that prevents any kind of Blitzkrieg
several threads have discussed this already ... but I don't mind contributing to a new one
