Old post, but I'll list off my army comp. I'll lead with the fact that this mod's units are all very powerful, but some have more niche uses that may not be as useful in as many scenarios as other units.

Leader unit: Franz, von Draken, Imperial General, Engineer, whatever
Imperial major: not required, but gives a good morale boost to nearby units + Master Maneuver makes microing units into flank positions a bit easier
Engineer: Several passive abilities boost the accuracy of artillery units in radius which is always nice and the Restock! ability is very helpful, especially during prolonged siege battles
Wizards of any stripe are cool, spells will always massively expand your toolbox.

I usually use 4-6 infantry, 2-3 machine guns (light or heavy), 1 fortress engineer, 2 units of either cavalry, armored trucks, or planes to protect the flanks, and the rest of my army I use artillery. Use a mix of field guns and howitzers, light and heavy. Field guns have a lower arc than howitzers, so AI units don't have as much time to dodge the fire. In addition, the shrapnel shells have a wider AoE than HE shells, making them very effective at destroying units of numerous but individually squishy horde infantry, as well as against massive/monstrous units. On the other hand, howitzers are better for firing over walls and fortifications, and poison gas canisters are very effective at area denial and destroying enemy artillery pieces. Heavy artillery hits harder than regular artillery, but has a slower rate of fire, so I like to keep a bit of a mix in my armies. Also, mortars just suck. Their rate of fire is a little bit faster than other pieces, but the arc is so high that their range suffers considerably and they'll have to lead their shots pretty heavily to hit moving targets. Unless you're playing on a lower difficulty where AI won't micro their units away from your artillery fire, don't use them. If you are playing on a lower difficulty, however, mortars become the artillery piece of choice during siege battles, regardless of whether you're attacking or defending. That high arc means there will be nowhere enemy units can seek cover.

the "core" o my armies is generally one fortress and base engineer, one unit of riflemen, and another of some flavor of Calvary then two artillery guns useally a HMG and a heavy feild gun. the engineers let me build up defensive guns and more units fairly quickly. The standard rifles let me put a nice blocking force if need be in between my base engineers and any fast harassing units while my mounted troops can harass the flanks. The artillery if loaded with shrapnel is disgustingly effective and will have no issue just deleting a unit or two before the foretress team has to start pulling back