Clearly, to do all of the units involved at Medina de Rio Seco would involve a substantial lottery win and a very big room to game it in. Neither of these is applicable at the moment, so I'll have to make do with creating a representative selection of troops.

My interpretation of the OOB for Medina is as follows (I've gone mostly with the Partridge and Oliver version). I have also commented on the availability of figures.

Part 1 - The French

Imperial Guard - nobody makes any of these in 40mm!
Fusilier Chasseurs - 2Battalions
Fusilier Grenadiers - 2 Battalions

1 Squadron each of
Chasseurs
Chevau-legers
Dragoons
Gendarmes (Oliver and Partridge suggest that the Guard Cavalry was only Gendarmes)

10 Guns

Merle's Division
47 Line - 2 Battalions (some sources suggest 1 Battalion)
2/3rd Swiss - 1 Battalion
14 Provisional Line - 1 Battalion
86 Line - 2 elite companies
13 Provisional Line - 4 Battalions
17 Provisional Line - 4 Battalions
18 Provisional Line - 1 Battalion

12 Guns

Mouton's Division
4 Leger - 3 Battalions (Leger not made in 40mm)
15 Line - 2 Battalions

6 Guns

Lasalle's Brigade (nobody makes a Lasalle figure in 40mm, which is a shame)
10 Chasseurs a Cheval
22 Chasseurs a Cheval

So some real issues here. Nobody makes the Guard at all in 40mm (Sash and Saber does Guard Grenadiers a Cheval, which could double up as Gendarmes at a pinch).

Nobody makes figures for Leger in 40mm, which could be just a paint job to make do, but I would rather have specific figures, there are some uniform differences from the Line.

A Lasalle personality figure would be great, as would a Bessieres.

The question is, how to get around the lack of availability of some key troop types.