Anything on horseback is essentially crap 2 (3 at most) decades into the game. Sure, the CAV w/ cool-guy brigades are fast and can mow down small-sized enemy formations, but they're expensive as hell and not really worth it. Plus they tend to die (take casualties) at an alarming rate, with the low defense ratings. So, A) they're very expensive, B) they deplete your soldier POPs like they're going out of style, C) they're offensive power is nothing to write home about... because most of the army techs are going to be boosting the infantry. So effectively the mounted troops become more and more obsolete, with each passing year / tech. By a certain point (and not that long into the game), they're just crap, all together. I let my horsies get down to just a couple hundred men per division, and then station them in places where I want to lower MIL, but not actually have to worry about an actual revolt (which would quickly kill them). They basically become a police force.
As for organizing the 'real men' army corps, it depends. Having a HQ division can be advantageous so that the corps will fight on, even if the others get low morale. Suppose you have a corps of 4 divisions; 3 artillery and 1 HQ... and you've got them well-entrenched in a mountainous region with lots of trees. Great defensive terrain. Then the enemy attacks you with 180k men. The battle lasts for months, and the enemy is fighting ineffectively, and taking huge attrition on the 1st of each month. Obviously, you want this battle to go on as long as possible... only retreating when you're RIGHT about to lose a division(s), which you don't want to lose. So, the HQ can make this happen. Their high morale will ensure that the enemy just can't seem to force you out of there, forcing you to fight to the last man (again, at horrible kill ratios, for the enemy).
But mostly, I try to organize my units so that no one is getting screwed over regarding their speed (speed is paramount). Thus, all the artillery tend to get grouped together - probably with a leader that increases speed... and they sort of function as the 'heavy armor' of the army (expensive, devestatingly lethal, but slow & carefully deployed). Then you might have an engineer corps... several divisions with engineer brigades -very tough defensively- which you use to hold the line in places where you want to put up a stone wall against enemy advancement... as you move out offensively, nearby.
Guards are just an infantry unit on steroids... they can be mixed with infantry, or whatever. Anywhere you want that 'extra-cool-guy' bonus, just put some guards in there, and they'll provide a slightly more effective fighting force.
Regulars, I use to increase the 'bullet sponge' effect... mainly for foreign troops. For example, "you can only create 6 divisions with (conquered) culture XYZ". OK, fine. Well guess what, all those divisions are going to have regulars. My way of putting more regular troops into the field, but still not exceeding the limit. Regulars are fairly cost-effective. It's literally 120% of an infantry division... just over-strength. But, it'll still bleed the same as regular infantry. It's not all cool-guy, like the guards (but, regulars have their uses, even for the national culture... like I said, they're pretty cost effective). So, use the tough defensive units (ART, ENG, GRD, etc.) for your own home culture POPs. Then put a bunch of foreign troops in there, with regulars. They'll take higher casualties. But, screw 'em. That's why we conquered them in the first place.
Vanilla infantry can have it's uses, when you stack a whole bunch of them together, like 1/4 million men. I just wrote a bunch of on this, in
this thread. Bascially in great (highly cost-effective) numbers, the sheer numbers themselves can give vanilla infantry a whole new attribute, on it's own. There is always something to be said for sheer numbers. I've seen 40,000 high quality troops (1 art, 1 eng, etc.) dug in very tight at a place that was full of hills and trees. But, when you attack with 1/2 million men, from several directions, suddenly no one seems to care how well you're dug in, or what the stinkin' terrain is. You will die -quick- and the large army will take very low casualties, compared to if it attacked with a more 'standard sized' formation.
But bottom line is, each of the units (namely ENG, ART) has it's own special attributes, and I like to concentrate them together, and capitalize on each of their strengths. 'Mixed units' do not really appeal to me. If you're an engineer, you should be with other engineers -- so I can put you in a certain place and form a really stiff defensive fortress in a sector of the front, so I can then use other units to go on offense, and not worry about that flank.
One last thing about CAV, they can be useful as late as the 1860s, when you group a bunch of them together. But even then, their casualties are too high, they're too expensive, and they're really not THAT much freakin' faster than the foot soldiers. CAV & HUS have limited uses in the game. A small number of them can be OK, but I seldom ever find myself 'recruiting a cavalry division'. Usually the ones I start with, are more than adequate.
