The official formats in the Pokémon TCG are Standard, Expanded, Theme, and Legacy, of which only Standard and Expanded are recognized in Play Pokémon events (the official Pokémon TCG events).
Pokémon Lv.X are not legal to use in any of these formats. Legacy has the most expansive legal card list, starting from the Heartgold Soulsilver set onwards. Pokémon Lv.X are only featured from sets Diamond and Pearl to Arceus.
You can, however, use both cards in some unofficial yet popular formats like Unlimited. In such formats, the most recent rulings on particular elements of the TCG are preserved, and otherwise the card text takes priority over mechanical rulings. If you did use a Pokémon Lv.X and a Pokémon Break in the same deck, you would be able to stack them so long as you placed the Lv.X first.
Pokémon Break can only evolve from a species with a specific name; for Example, this Empoleon BREAK can evolve from Empoleon, but not Empoleon ex, Empoleon EX, Empoleon GX, Empoleon V, Empoleon VMax, Empoleon VStar, Empoleon PRIME, or Empoleon BREAK. This is because the Pokemon who Empoleon BREAK must evolve from's full name must be "Empoleon".
However, Pokémon Lv.X’s names are treated as if they didn’t have the “Lv.X” part. If you went the other way, however, the same logic can be applied to Pokémon Lv.X. Empoleon Lv.X says "put onto Empoleon", meaning it cannot be put onto "Empoleon BREAK".
Do note that since Poképowers and Pokébodies were out of rotation when Pokémon Break were in, so Pokémon Break do not inherit them from their evolution card underneath. (So Empoleon Lv.X in the above Example could not use it’s Supreme Command Poképower if you placed Empoleon BREAK onto it).
Hope I Helped!
Sources: 1, 2, 3
