This may be taken as somewhat typical of the information included in a warrant to raise a regiment or additional battalion:
"Whereas we have thought fit to order a Regiment of Fencible men, to be forthwith raised under your command, which is to consist of ten companies, of 4 sergeants, 5 corporals, 2 drummers, and 95 men in each, with 2 fifers to the Grenadier Company, besides a sergeant-major and quartermaster-sergeant, together with the usual commissioned officers; which men are to serve in Great Britain and Ireland only."
In some instances, how the officers were to be appointed or obtain rank in the regiment [whether to be taken from the half-pay list or by recruiting for rank] and whether or not half-pay privileges would be extended were also included in warrants.
Every year, the estimates for the army were presented in Parliament and here was the opportunity for the establishments to be changed, as well as anytime during the year if it was thought necessary even after the original estimates were approved.
The Guards were a special case. I believe that the higher establishment may have been necessary to ensure that sufficient men were left in Britain to guard the sovereign whenever some of the battalions went overseas. The second battalions were the home service battalions of the regiments for the most part, although some companies did serve overseas.
The Guards regiments always had a higher establishment of officers. This was due to the fact that the regiments only ever had one regimental Lieutenant Colonel and the other battalions were commanded by Majors. Due to their seniority in the army the senior positions of the regiment were held by General Officers or Colonels, who were usually on staff somewhere and so whenever a battalion went on active service it was commanded by the 11th Captain or so who assumed the rank of an acting Major in the regiment [they would of course hold the dual rank of Lieutenant Colonel in the army overall]. Until 1814, these senior officers were allowed to hold their commissions in the Guards even when promoted to General Officer's rank.