Under no known biology laws is the mating between two alien species possible. We humans can't even mate with gorillas, that are the closest thing to us. Even considering that, even with extraordinary technical and genetic abilities, like the ones the "First Ones" have, it would be extremely difficult to produce beings with so much resemblance to one another. There could be one with three eyes, another with their nose in the forehead, another with their mouth above their eyes, small ones the size of an apple, or tall ones the size of a building. Most of them would probably use very different mating methods than our two-gender tube-in-the-hole one. It would be impossible to even consume alien foods, for that matter.
For things like the above to be remotely possible in the real world, the "First Ones" would have to colonise the other planets with their own flora and fauna, and then program their evolution.
But we're talking Star Trek here. Where parallel worlds whose people speak English exists in our own Galaxy. So we should just consider it poetic licence.