The two best ways to keep feeder clams alive are these:
Put them in a separate feeder tank with no sand or other substrate on the bottom, so that they can't bury themselves.
But them in one of those little net boxes that fit into your tank - they're sold for breeding or isolation and hang into the water from the top of the tank. Put the clams only one deep.
Did you buy something to feed the clams with from Fish Supply?
Small hermit crabs would be a good choice for your bimacs' first live food. Maybe some smaller snails, too. Then you could move to small fiddlers, if you can get them or wish to mail order them. (I clip the tips of the claws of the males.) Ghost shrimp are freshwater, as you know, and not too nutritious, but they're good practice for the little guy when he gets old enough to catch shrimp. They're much slower and easier to catch than any feeder saltwater shrimp I've seen.
Don't know about those red crabs, but generally you should try for live food that lives in saltwater.
I believe they will take small pieces of frozen, thawed shrimp or fish when they're 2 months old.
Let us know when he arrives. Do you have a name picked out yet?
All the best,