What values do starburst and giant symbol units need in order to break away from a colossal? Also, does phasing/HSS/leap climb all work the same when faced with a colossal figure?

There are no special rules for a character trying to break away from a colossal. Baring any powers they might have, break away would still occur on a 4-6. HSS and Leap/Climb would both break away automatically still, and Phasing/Teleport would still ignore the colossal when they move.

This is not entirely correct. Some of the older colossal figures do have special breakaway rules:

Fin Fang Foom

Fin Fang Foom automatically breaks away from other characters, and other characters automatically break away from Fin Fang Foom unless he has Plasticity. No break away involving Fin Fang Foom deals damage.

The Spectre

The Spectre automatically breaks away from other characters, and other characters automatically break away from The Spectre. No break away involving The Spectre may deal damage.

Galactus (both of the original ones)

Galactus always succeeds at break away rolls, and opposing characters always succeed at breaking away from Galactus.

Dark Phoenix

does not need to roll to break away, although characters adjacent to big figures roll for break away per the standard rules;

Dr. Manhattan

Dr. Manhattan automatically breaks away from other characters, and other characters automatically break away from Dr. Manhattan. No break away involving Dr. Manhattan deals damage.

You forgot Apocalypse. He has plasticity. But that is why I included the thing about "barring any powers they might have." Being colossal in and of itself does nothing about characters trying to break away, though.

Although I am very close to making a signature finally. "Everything I posted is for general cases. Some characters might break these rules."