If I have two Rainbow energy attached to a Dark Charizard and I flip two heads, can I discard the Rainbow Energy or do I have to have Fire Energy cards?
You could discard the Rainbow Energy as they do count as all basic energy types.
If a card gives off Energy of any type then that card COUNTS as an Energy card of that type. This means a Buzzap'd Electrode that was chosen to give 2 Grass Energy COUNTS as a Grass Energy Card and can be Energy Transferred by Venusaur's Pokémon Power.