In your article Do Angels Have Wings?, you state that seraphim and cherubim are angels and do have wings. I find no reference in Scripture that refers to them as angels, so I would like to know on what basis you make that conclusion.
They are generally considered angels just because there are a limited number of intelligent sentient beings. The general categories are God himself, angels, and human beings, and some would also include higher animals.
The “living creatures” and seraphim and cherubim are normally considered different types of angels. They are not called angels per say so it’s fine to think of them as non-angels, but if they fall into any general classification, obviously they are not humans and they are not God, so angels is the natural grouping. But it really doesn’t matter what they’re called.
Here’s an article about seraphim.