If you pay {W}{U}{B}{R}{G} rather than pay the mana cost of a spell with {X} in its mana cost, the only legal choice for X is 0.

Fist of Suns's ability is an alternative cost to cast a spell. You can't combine this with other alternative costs, such as flashback. You can pay additional costs, such as kicker, in addition to this alternative cost.

If you apply Fist of Suns's alternative cost to a spell with {X} in its mana cost, X is 0.

If you cast a spell for which mana of any color can be spent to cast it, you may cast it for Fist of Suns's alternative cost and still spend any color of mana to cover that cost.