I tend to build corps (divisions in ACW 2 terms) composed of 2 infantry divisions (when you build them, 1 division = 2 Inf reg + 1 field art reg) and 2 med or field art. Which gives 1 corps = 4 inf/mil regiments and 4 artillery. Stack two corps like these with a ** general and you have a nice independant corp with a fire power of about 500 and no CP penalties.
For armies, I stack 2 of those "doublle corps" with a *** general, add in transmission and balloons, 1 cav, and whatever heavy or super heavy art can fit without going over the CP limit of 48.
I think that in case of a siege, one can add extra super heavy artillery even if it gives something like 5% CP penalty.
As WE, I usually buy all infantry, field artillery and med artillery as soon as possible (I nearly exhaust the pool), merge them all in one province once they are ready, deploy there all my ** generals, and build my corps from here.