I've had a similar idea regarding some of the races. Its possible assuming the aliens we see here are the only species that live close to the planet, which has a lot of satellites according to Ohara.
But there might be some issues. For one, I don't think the CDs wear the helmets because of that since they're said to not be different from humans at all. They probably wear the helmets because of arrogance or some other conspiracy since neither Imu or the Gorosei wear them.
If I were to take a wild guess...
I'm guessing the Lunarians were the original inhabitants of the moon who first settled on the OP world's Red Line. Then their descendants who settled outside of the Red Line changed over time and became the first humans while the Lunarians on the Red Line kept their wings, fire power and skin color. The humans eventually rebelled against their pureblood Lunarian forebears and claimed the Red Line as their own, driving the terrestrial Lunarians to extinction as revenge for whatever reason and founded the World Government. The remaining Lunarians that stayed on the OP moon kept evolving and split into the three tribes with smaller wings and no fire power as resources ran dry and who then went down to the world below once resources on the moon disappeared and colonized the Sky Islands and Jaya.
The Fishmen could be aliens, but the fact that they're still considered humans by some makes me think that they're simply humans who took on their current appearance after their home island was sunk beneath the Red Line for whatever reason and since it was in a bubble provided oxygen from the surface, they probably adapted over the centuries as they swam outside their bubble, eventually becoming fishmen. Either that or that whale alien's ancestors went down to the OP world and hybradized somehow with the sunken humans; I admit we've never seen whale fishmen before, but there's mollusk fishmen and that one orca fishman from the 4th OP movie who was designed by Oda.
I like to think the Minks are definitely aliens with how they seem to have an affinity with the moon and their weird electric powers, so the Fox and Panther aliens may be part of a greater alien Mink race that has foxes as the dominant species, and foxes in yokai culture are said to take on many forms, so these fox aliens evolving or transforming to look like different kinds animal-like aliens would fit that theme I guess.
Then there's the two weird-looking humanoid aliens with facial hair. There's a chance they're the actual ancestors of humans who overthrew the Lunarians I guess which would fit with what you said before, but even with bubble helmets and all, they weren't immune to the changes brought about by the OP world's atmosphere which made them look like regular humans but with some abnormalities. The bubble helmets did probably help partly though since the CD all have the same height and less weird appearances compared to surface dwellers.
Finally there's the chicken alien and the crocodile aliens, but so far we haven't seen any birdmen or lizardmen in One Piece. I thought Morgans would be a bird dude and a possible descendant, but lOda just made him a devil fruit user for some reason. So there goes that idea.