Tank construction is the same between fresh and salt, so the weight loads you can have are the same. Water already weighs 8lb/gal. Rock is denser, but not tremendously so, and there's a considerable safety margin built into the glass. I doubt you'll get anywhere near the limit before you get tired of the rock pile obstructing your view of the fish.
The egg crate (not the same as egg carton - it's basically a hard plastic mesh) would be for the purpose of evenly distributing the weight of the rock across the entire tank bottom rather than having it rest on just a few points of contact. Essentially any hard flat surface would do, but egg crate is a lightweight material and known to be aquarium safe. It just serves to support the rock, distribute weight, and absorb shock if the rock shifts.