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If a card in a library has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

If a spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Since you are using an alternative cost to cast the spells, you can't pay any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.