A few helps, though it's impossible to answer all your questions as fully as I'd like. I'm sure some folks can recommend some good reading for you.
1. Yes, units are battalions. One battalion can carry the eagle, and the other three fanions. I'm not sure what list you're using, and I don't have the Salamanca OOB in front of me here, but you could also divide your four battalions into two regiments of two battalions apiece, with each regiment having an eagle-bearing battalion and a fanion-bearing battalion.
2. As for how many battalions LaSalle specifies per regiment, you won't find that in there. That could vary by campaign and theater. Some French units in 1812 went into Russia with four or five battalions; French units in 1815 took the field with as few as two (if memory serves correctly). If you're going by an historical OOB, then you can follow that. If I have a LaSalle list with six core battalions, I can build one regiment of four battalions and one of two, or two regiments with three battalions, or even two regiments with two battalions and two regiments with only one battalion apiece. In Spain, especially, battalions often served far away from their parent regiments. Single-battalion "regiments" were not uncommon.
3. I don't usually build specific OOBs. I field a regiment I like, and "plug" it in to a scenario if I need it. If I have painted the 45th Ligne with three battalions, and I play a scenario calling for the 39th Ligne with three battalions, I just plug the 45th in and rate them as necessary. A battalion is a battalion is a battalion, in my eyes.
4. Veteran vs. conscript? That knowledge will be based on your understanding of the historical battle being portrayed (if you're sold on portraying a specific battle, that is). If you're interested in Salamanca, only doing some thorough reading on that battle will answer those questions. And once you've answered them, you'll run into gamers who have their own opinions. You'll soon see that opinions are like – well, you-know-what – everyone has one.