Megas in general are not very good right now, this is mostly because
1. Field Blower makes it hard to keep spirit links on
2. GX cards have largely powercrept them, and the lack of access to choice band really hurts their damage:energy ratios compared to GX cards
3. Rotation took away mega turbo which was particularly important for energy scaling megas like Gardevoir and Mewtwo
In general, you really don't want to run Mega Garde with a second type of basic energy because its damage only scales on fairy energy, and requires 2 attachments to attack. Dual energy type decks are pretty uncommon in general due to the inconsistency they introduce, only Tapu Bulu/Vikavolt, Ultra Necrozma/Malamar and some Zoroark variants really run multiple types of basic energy.
I recommend considering Xerneas BREAK as an alternative, as it has a very similar attack to Mega Gardevoir-EX PRC, has built in acceleration from Xerneas XY, and is also a one prize card. As for fighting type partners, I recommend Sudowoodo BKT with Counter Energies. Counter Energy is great because when you're one prize down (which will often be the case in a deck with a one prize attacker) it counts as 2 of any energy, both allowing a one turn powerup of a Xerneas and allowing certain tech cards to work. On Sudowoodo, it can let you copy a Zoroark's Riotous Beating to return an OHKO to it. Baby Buzzwole is another counter energy friendly option, and you can also include a Mewtwo EVO to nab revenge kills on psychic weak things.
Oranguru SUM is key for Xerneas BREAK as is Max Elixir. You should run at least 10 basic energy, ideally 12 to have a good shot at elixirs hitting, you'll want some DCEs and counters since Xerneas damage scales from all energy on your board. Don't get too creative with the draw engine, I'd go with the standard N/Sycamore/Cynthia/Guzma supporter lineup, then standard items like Ultra Ball, Choice Band, Float Stones, and a couple of Field Blower and Rescue Stretcher. Float Stones can be worked around a bit if you run Fairy Garden as your stadium, or you can choose to run a few Parallel City if you want to put the hurt on Zoroark and Necrozma decks, which flounder without being able to run a full bench.