You are mistaken.
The paper strength of a regiment was around 2-3,000 men, divided over two or three battalions. Can't remember the exact British practise but IIRC it's one or two battalion for active duty, 1 for depot duty (recruitment and training) The battalions weren't all doing the same thing at the same time. It seems to me that the 1st and 2nd battalions were typically not used in the same campaign if it could be avoided. A regiment was, in most armies, an administrative unit only. Battalions were the unit of manoeuvre. Therefore, in the Total War context one might wish to use the term battalion for a single unit.
Battalions were ideally 800-1,000 strong, but only favoured regiments (Guards mostly) managed to get close to their full strength complement in the field. Typical battalion strength seems to have varied wildly, depending on circumstances. The 400-600 range seems to have been common. Lower if it's a long drawn out or bloody campaign. Therefore, a TW unit at a full strength of 500 men would be a very fair representation of a full strength battalion in the field.
If that's a little too much for your PC but wish to keep to a realistic unit scale, you'll have to go down a level of command to the component parts of a battalion, the company. A battalion typically consists of 10 companies. These companies tended to stick together with the battalion forming the component parts of a single formation, bar the light company which was more flexibly used, most notably the skirmishing role.
If you want to have a realistic battalion, you have to use one grenadier unit, one light infantry unit (not rifle) and 8 units of line infantry, each 40-60 strong. Or, to save unit slots, 1 light and 1 grenadier @ 40-60 men and 4 line units @ 80-120 strong.
Throughout much of the era depicted by Empire many nations issued their battalions a pair of cannon in the 2-4 pounder range as well.