The beauty of the rules is that they play well with a brigade up to multiple corps level. As for the composition of your army, I would suggest you simply base it on OOB from whichever army you choose to paint. You will then get the correct balance of Infantry, Artillary and Cavalry.
This is the only set of rules I have played where I don't have the urge to tinker with the rules, the command and control element plays fantastically well (especially for larger games).
Simon