If I use Mew VMAX's "Cross Fusion Strike" attack and choose an attack with a cost of more than just {C}{C}, can I do it even if I only have two of any energy attached to Mew VMAX?

Yes, you only have to pay the attack cost of Cross Fusion Strike. Ignore the attack cost of the copied attack whether it is more, less, or the same; regardless of the type or amount of energy required for the copied attack.

Relevant cards

SWSH8_114