The Voices of Liberty have a daily schedule, usually every 45 minutes starting at 12:15 and the last show at 4:45 (this can change but that's currently the schedule). I would get there early (maybe 15 minutes ahead of the show) unless you don't mind standing (there's not really a lot of seating, we usually just stand).
American Adventure is also on a schedule, 45 minutes apart from 11:15 to 9 PM. If you wanted to do both together we usually pick a Voices of Liberty time and then see the American Adventure right after. As an example the 12:15 Voices of Liberty followed by the 12:45 American Adventure. It's easiest to pick a time for Voices, arrive 15 minutes ahead of that and expect to be there an hour after the Voices show time.
ETA: Just saw your 6 PM time, and that won't work. The Voices last show is 4:45.
There is plenty of room to see them, but seating is a few benches in the Rotunda and I'm not sure how early you'd have to be there to sit. We usually duck in about 15 minutes early and are able to either sit further back on the few remaining benches along the wall, or just stand behind others. Many people sit on the floor as well.