Are you adding damsels because you want to keep them forever, or because they are hardy fish and you want to remove them later?
Damsels should be the last fish added as they tend to bully any new fish introduced.
Why wait for the sump? It would be best to have all your equipment already in place and running (tho I wouldn't run a PS during the cycle), to make sure you don't have major problems after you add livestock.
Crabs generally kill and eat snails, so be prepared to keep restocking.