If you get both snails and crabs, the crabs will most likely eat the snails so they can have their shells. Some people believe that, if you put plenty of empty shells in there, the crabs will leave the snails alone. Others swear left and right that the crabs WILL eat the snails. I'm not adamant either way, but I had both in a tank and was left with only one nassarius snail after a few months. Your call.
Not sure at all about the antler. If you put anything at all in your tank, though, you have to boil it beforehand in order to eliminate infecting your tank with something. My guess would be that this process may remove all the good calcium, as well. This is completely a guess, though. Best of luck, Luke!