Hi and welcome to TONMO.com!
Just cycle your tank - will take three months or so. You should have a nice build up of amphipods that your future octo will like. Before your octo arrives, you can add snails, hermit crabs and other members of your clean up crew that your octo will also enjoy.
No need to get the clams early - in fact, if your order them with your octo, you should keep them in one of those little breeder or isolation net containers that fit into the tank. This is so you can monitor them and keep them from burying in the sand and dying.
Also, you don't need quite such a deep sand bed - 1-2 inches will do.
Nancy