For the crush attack, I made it a full-round action rather than standard, since crush is usually for flying creatures, and H'Calos actually has to rear up. But I don't see any reason it couldn't be a standard action for H'Calos as well.
Crush: DC 28. The save DC is Constitution-based. Deals 4d8+24 damage.
Frightful Presence: DC 21. The save DC is Charisma-based. Radius - 200 ft. (as mountain landwyrm) or 60 ft. (as tarrasque) or something else?
Rend: Deals additional 2d6+24 points of damage.
Swallow Whole: Looking at similar Gargantuan and Colossal creatures...
-The tarrasque’s gullet can hold 2 Huge, 8 Large, 32 Medium, 128 Small, or 512 Tiny or smaller creatures. (Colossal)
-A Gargantuan fiendwurm's gizzard can hold 2 Huge, 8 Large, 32 Medium-size, or 128 Small or smaller opponents.
-A cloud ray's throat can hold 2 Huge, 8 Large, 32 Medium-size, or 128 Small or smaller creatures. (Colossal)
-An ulgurstasta's maw can hold 2 Huge, 8 Large, 32 Medium-size, or 128 Small or smaller opponents. (Gargantuan)
...I'd say we can safely go with 2 Huge, 8 Large, 32 Medium, and either "128 Small, or 512 Tiny or smaller creatures" or "128 Small or smaller opponents".