Marc, if you're interested in a historical basis to which pre-war regiments you choose, at the start of the war the small Spanish army was heavily concentrated in 4 or 5 places – there were hardly any troops anywhere else (except Guards and Swiss around Madrid):
(A) Galicia (Ferrol + Coruna the naval bases, defending against possible British attacks): this became the Army of Galicia and Blake the commander did NOT fill up his veteran regiments before moving them to the front, so they all wore their nice uniforms, like the Romana Division. Recruits were put in new regiments, which Blake left behind in the summer campaign (Medina del Rio Seco).
(B) The Romana Division, which cameronian gave details mentioned, was in Denmark but was rescued by the British fleet (except Guadalajara, Asturias and the Algarve cavalry) and delivered to the Army of Galicia in time for the Battle of Espinosa in November (where they did very well indeed). The infantry that got home – and to Espinosa – were:
-Zamora (3 batts) 54 offs 1757 men
-Princesa (3 batts) 62 offs 1953 men
-1st Catalonia (light: 1802 uniform): 44 offs 1066 men
-1st Barcelona (light: 1805 uniform, shako, green side plume) 39 off 1205 men
(C) The Army of Andalucia (under Castanos) blockading Gibraltar – and Castanos DID fill up his veteran regiments, each veteran keeping his bicorn and jacket and handing over to the recruit his fatigue jacket (white with facings colour on colour and cuffs and forage cap (nightcap style, white piped facing colour).
(D) the small garrison of Majorca (defending against the British who had pinched Minorca when they took Gibraltar) – who were shipped to Catalonia (wearing their nice uniforms) for the Battle of Cardadeu (December)
– plus finally (E) the army occupying Portugal that either got captured by the French or evaded to join either the Army of Galicia or of Andalucia – so are irrelevant.
So 4 options in total (A-D), 3 options with fully regulation uniforms: (A Galicia, B Romana, D Majorcans in Catalonia). The latter a side show really.
The pre-war regiments in the Army of Galicia at the Battle of Medina del Rio Seco were: – figures in brackets are the battalions/companies:
-Zaragossa (1802 uniform: 6 coys – probably fusiliers, not clear over grenadiers) 700
-Hibernia (Irish regt 2nd+3rd, so 8 coys of fusiliers) 1200
-Majorca (6 coys of fusiliers) 1000
-Rey (2nd+3rd so 8 coys of fusiliers) 700
-Principe (8 coys of fusiliers) 1400
-Toledo (10 coys of fusiliers) 1600
-Aragon (6 coys of fusiliers) 1068
-Navara (10 coys of fusiliers) 1579 (not at battle)
-Sevilla(10 coys of fusiliers) 1577 (not at battle
-Naples (Uniform like Irish: 10 coys of fusiliers) 1960 (not at battle)
-Vol de Navarra (Light) (4 coys) 754
-Vol Gerona (Light: 50%) 400
-Vol Barbastro (Light 50%, 1802 uniform) 550
-2nd Vol Catalonia (Light 50%) 400
Lots of militia – who wore brown jackets, white breeches, red lapels and facings – not the white jackets one usually sees.
United grenadiers in Vanguard Division: 2 batts of 800 total (400 each) with 8 coys, (so 2 batts of 4 coys at 400 each) with the 2 grenadier coys from each of following: Zaragossa, Mallorca, Aragon, Vols de la Corona.
United grenadiers in 4th Division: 2 batts with 10 coys total 1,100 (so 5 coys each, 550 per batt?) with the 2 grenadier coys from each of following: Principe, Toledo, Sevilla, Navarra, Naples.
Source: Arteche's 1875 Guerra de la Independencia, Vol 2, Appendix + Appendix B to 2008 edition.
When the Romana Division was in action at the Battle of Espinosa (November 1808) they do not appear to have detached their grenadiers and the line up when they got back from Denmark was:
-Zamora 3 batts 54 offs 1757 men
-Princesa 3 batts 62 offs 1953 men
- 1st Catalonia (light 1802 uniform) 1 batt 44 offs 1066 men
– 1st Barcelona (light: 1805 uniform with shakos, plume on right side 1 batt 39 offs 1205 men.
Source Arteche Vol 3 Appendix 10.
Hope this helps you choose your regiments.
Choose the ones with the facing colours you like!
Best wishes