Strago and Relm

If a spell has {X} in its mana cost, you must choose 0 as the value of X when playing 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. If the spell has any mandatory additional costs, those must be paid to cast the spell.

You cast the card while Strago and Relm's ability is resolving and still on the stack. You can't wait to play it later in the turn. Timing restrictions based on its types are ignored.