So, I have thought about the evolution of my aliens and how they could evolve things like lactation but still retain their reptilian characteristics.
Here is my best thought about it so far:
500 million years before
A lizard like animal crossbreeds with a kangaroo-like animal. This diverges on 2 paths, 1 where reptilian characteristics are lost and another where they are retained but a pouch is added. The mammalian branch of this crossbreed dies out because in their environment, being cold blooded is advantageous.
400 million years before
This still lizard-like animal that has a pouch crossbreeds with an intelligent biped. Now being bipedal is retained along with the rest of the characteristics(the pouch and reptilian characteristics).
300 million years before
This bipedal reptilian crossbreeds with this time a full blown mammal, not just an animal with mamallian characteristics that doesn't lactate. Now lactation evolves. Around the same time milk proteins specific to sleep schedule evolve. They start to lose their scales and have bare skin in place of it and even a little bit of hair.
200 million years before
This almost half-mammal half-reptile creature crossbreeds with another reptile. They lose all but the lactation and previously retained traits. Now have fully scaly bodies again. Also at this point in time, kidney-like organs have evolved. These organs only allow m1 proteins in the blood vessels supplying the higher nipples, allow both m1 and m2 proteins in the blood vessels supplying the middle nipples, and only allow m2 proteins in the blood vessels supplying the lower nipples in a similar way to how kidneys selectively secrete and absord different substances in different areas of the nephron. So what nipple an alien suckles from determines their sleep schedule after they hatch.
100 million years before
All that is left to evolve is sixth digits. This proto-alien crossbreeds with a bipedal lizard-like animal that has 6 digits on every hand and foot and has the outer 2 digits on the hands more like thumbs.
So after all that you have the alien species.
Impossible? I don't think so
Now I have been told that this is impossible because mammals are too far diverged from reptiles to crossbreed. But there was a point in time when there were mammal-like reptiles and a point in time when there were reptile-like mammals. So clearly, millions of years ago, the difference between reptiles and mammals was slim. This slim difference could mean potential crossbreeding.
Here is what I think happens when the aliens evolve:
Reptile x Mammal-like Reptile(warm blooded, scaleless)
Reptile x Reptile-like mammal
Mammal-like Reptile x Mammal
Reptile-like Mammal x Reptile
Mammal like Reptile x Reptile
Mostly reptilian with just a few mammalian characteristics
Is the evolution of my aliens plausible the way I wrote it down?