Yeah, you summed it up good. Things like this are compromise between AI, game mechanics and historical authenticity... Having general an infantry unit, removes lots of exploits, besides presenting best way how to simulate Grenadiers in a century, where each regiment had its own company of Grenadiers and Grenadiers were never forming separate regiments. They were occasionally formed into battalions, and if you do the math, with 20 units limitation, you get about 1.5 battalions of Grenadiers if you only have infantry, or 1 battalion, with properly balanced army.