Will,
Yes, fiberglass panels are excellent. Straddle the corners with them and checker board a couple of the walls. Rule: never let untreated surfaces face each other. Absorption panels, diffusors, geometric shapes, angled panels, etc all count as acoustic treatment.
If the ceilings are angled and higher than 10 feet, try leaving them alone for the time being until you can do some tests. You don't want to over-treat, but you want to bring the decay time down to around .6 to .9 seconds.
Cheers,
John