You cast the copy while Shiko’s last ability is resolving and still on the stack. You can’t wait to cast it later in the turn.

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

If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.

If the 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.

If you don’t want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are checked.

A resolving copy of a permanent spell becomes a token. That token isn’t “created” and won’t interact with abilities that care about tokens being created.