What kind of snails? Astreas are famous for not being able to "right" themselves if tipped on their backs.
My first suspect would be acclimation procedures. Most mobile invertebrates are pretty senstive to changes in pH, salinity, & temp.
If you could provide some tank parameters, that would be greatly helpful -- temp, salinity, etc. I've always and forever run our reef tank(s) at 1.025 at 80F - 82F. Have never had a problem. Low salinity is often an issue, but more in the long-term range. Again, I'd first suspect acclimation.
I have a Copperband and the fish is awesome. It'll eat any feather dusters you might have, but will also chow all Aiptasia. I really don't have much for softies in our tank, so I can't speak for compatibility with those. They can be really difficult to get to eat though, so keep an eye on it.
The Arrow crab would worry me the most - they are notorious for not only eating all of your bristle worm population, but hermit crabs as well if you have any.
As far as the Coral Beauty goes - most dwarf angels are pretty risky as far as coral-nipping tendencies, but they tend to prefer LPS
(Open Brains, etc.).
Good luck with everything, and don't stop asking questions!