Regular infantry has more fire power and ability to hold ground and ability to face heavy targets. Light infantry however has a bit more organization to represent they're more mobile in battle, has more movement and ability to fight on irregular terrains but at the cost of firepower and defensive ability. Of course doctrines such as guerrilla could make them even better.Thats some good ideas you have there!
What will be the differences between infantry and light infantry?
Go faster than the full divs and are weaker?
Likely yes, they'll also fit the role of mountain troops such as Bergsjägers but also represent the famous french foreign legion.Will your Elites=Guards=SS Troops?