1. Is crouching never allowed or just while in active combat? How about for double rank lines?
2. For skirmishers, is it 6 rifles + 9 Light infantry when you sign up or is it just one or the other?
1) If your line is in active combat there is no crouching. General non-combative crouching is fine (hiding below a ridge, hill, or just a quick indication you are AFK are all acceptable). Double rank lines allow for crouching.
2) Its both so your unit can max at 15 (excluding the officer). As for the division of who gets what class, that will happen internally for whichever regiment is indicated as the Lights/Skirms reg for the night.