If it's from a rotated set, can you still use (basic) Energy in a deck for an official event? I'm wondering if I can use the holo Energy cards from the old ex series.
Yes, Energies are basically reprints of Base Set Energy. You can use any Basic Energy Card in an official event. To answer your question yes, you can use holo Energy Cards from the Old ex Series.
A rule of thumb: all Basic Energy cards are always legal, unless they're foreign of course. I use primarily Player Rewards Energy or old Base Set Energy for all my decks.
