I need to choose what regiments to use in my army

What Regiments do I use?

The last thing I need to do for my army is put on flags - and to do that I need to decide which historical regiments I want to represent.

I started the army based on the  Battle of Fuentes de Oñoro.

I know I want a battalion to be the South Essex (Sharp's fictional regiment) but can't find their flag in the right scale, so might do one of the other Essex battalions. One of them is a Highlander unit, and I've got the 95th Rifles...who don't use one being light infantry.

This ancient post on TMP suggests they were going to use:

For a while I have been dithering about which Napoleonic army I should paint to face my growing 28mm French army. I had been contemplating using Spanish, British or even Dutch, but couldn't decide.

Well, I have now come up with an idea. The army to face my French will be ... Richard Sharpe's army!

In other words, I'll do an army based on the Richard Sharpe novels by Bernard Cornwell. This could include both fictional and real-life units, such as the British South Essex Regiment (fict), 33rd Regiment of Foot (1st Yorkshire West Riding), 74th Highlanders, Royal Marines and the 95th Rifles (2nd Battalion); Spanish such as the cowardly Regimentia de Santa Maria (fict) and the somewhat better
Garda Real Irlandsa (sp? - fict), plus, of course, lots of guerillas; Portuguese cacadores; Kings German Legion cavalry, American privateersmen - and many other various units that Richard Sharpe belonged to, fought with or that otherwise crop up in the books.

Basing my army on a fictional novel series will give me freedom to paint lots of interesting units that in a real army would probably not have been brigaded together.

 And I need a rocket battery in 15-18mm, that no one seems to sell. 

I do want to use the 23rd Light Dragoons who charged impulsivity into a ravine at Battle of Talavera.

I am also constrained in what flags are available in 18mm.