So... Can the Guardians of Peace (or any Protect unit with 2 damage resistance) use their keyword to save another unit from Force Storm before dying or they all take simultaneously the damage and no protection can be made? In Faqs 2.8 says the active player decides order in case of conflict between 2 simultaneous effects that collide, but I am not sure if there is such "collision" here given there are no opposed effects but maybe "stacked".

Thanks in advance.

The damage is dealt simultaneously, but protect cannot be used to assign damage to the protecting unit beyond their damage capacity. In the case of effects like Force Storm, Force Shockwave, or Admiral Ackbar this means that the sum of the incoming damage to the unit with protect from the original effect (Force Storm, in your question) and from damage being protected cannot exceed the remaining damage capacity. (Note, the original effect on its own may exceed the remaining damage capacity). So if the Guardian of the Peace isn't committed it will not be able to protect any damage away from another unit.