The organisation of the Imperial guard reflected the traditional organisation of the French Royal army, that is 16 units combined in 8 companies; The grenadier company of the line regiments was in fact a levy in kind, a tax, imposed on the captains of each company; the captains were responsible for providing the men in their command. It had nothing to do with tactical organisation. Reason why the grenadiers were not normally with the parent battalion.
The regiments étrangers of the Royal Army (one third of the total strength) were traditionally raised with 6 fusilier companies of 120 rank and file, sometimes even 176 men. They used the same drill. So obviously, you are right that it doesn't really matter how many units there are, as long as you can divide the manoeuver units (sections) by four.