Go with a shallow substrate...like 1" or so. BB just doesn't look "finished" and if you get any bottom dwellers, they will do better with something to rest on.
As for a 29...it would work with a dwarf lion, but it's the bare minimum IMHO/IME. Be sure to give it a cave/overhang or two.
The lion won't eat snails or hermit crabs, but non-hermits and shrimp will become food. It also may take down a blenny. I'd add the lion first and get it established before adding any piscine tankmates, but the lion will be a pretty heavy load for that setup, so you'll have to work to keep your water quality up.
Here's a pretty good article on keeping lionfish that you should read:
http://www.lionfishlair.com/careguides/lionfish.shtml
Lions are great fish, and I highly recommend going with a dwarf fuzzy over the zebra, esp. for a first lionfish. Fuzzies are a lot more hardy and are easier to wean. You'll have a little aquatic puppy dog in no time!