If I use Charizard's "Recall" attack to use Charmeleon's "Slash" attack, do I need to have the required energy cost to use "Slash"?

No, Charizard just needs the one energy required to use Recall. You do not need to meet the energy requirement from the previous Evolution's attack, but if that attack requires you to do something with energy (like discarding an energy card) you must follow that attack's instructions.

Relevant cards

G1_RC5