Necrozma is described as absorbing the "light" from Lunala and Solgaleo. This is supposedly the same "light" that created Mega Crystals and such.
Therefore, could Necrozma absorb other Pokémon, such as Rayquaza? Not only is Rayquaza legendary and therefore would have more of the life energy that created the Superweapon (that created the Mega Crystals), but Rayquaza then ate an asteroid full of Mega Crystals. That's why Rayquaza can Mega evolve on command.
Therefore, can Necrozma absorb other legendaries and mega Pokémon?
Note, I'm asking about the plot/lore of Pokemon, not gameplay-wise.
Best Answer
First, to clarify, Mega Crystals do not exist in Pokemon lore. The OP appears to describe this as a false amalgamation of Z-Crystals and Mega Stones, which are completely distinct items.
Mega Stones are focused on in Gen 6. They are described as originating as Evolution stones irradiated by either Xerneas or Yveltal. Rayquza does not need a Mega Stone to evolve, but rather is able to use the energy from consumed Meteorites to Evolve.
Z-Crystals are focused on in Gen 7. Though the origin of these items is unclear, Z-Power has been linked to the energy from Ultra Wormholes and Necrozma's light. This is not clearly the same light that Lunala and Solgaleo emit, though they are capable of generating Ultra Wormholes.
Returning to the question, though Necrozma can fuse with Solgaleo and Lunala, there is no known record of it ever fusing with another Pokemon or being capable of doing so. Necrozma is capable of absorbing light, which may explain why it can fuse with Solgaleo and Lunala as they emit lots of light. However, absorbing light does not necessarily mean Necrozma can absorb Pokemon that emit light. This is a separate phenomena from Mega Stones, and I would not assume that it can absorb other legendaries and Mega evolutions on this basis.
I would assume Necrozma cannot absorb other legendaries and Mega Pokémon, simply because there is no record or definitive evidence to suggest it can.