Is it considered against the rule to upload arts you commissioned and paid for, if said art is not made publicly available by the artist? Assuming the artist (or skeb, as I can't find anything in their terms that prohibits this) said nothing against this, and credits are given. Same for arts where the artist uploads the art with watermark publicly, but I have the version without?

And do artists generally frown upon this?

edit: This does not apply to artists who post the art to exclusive places like Patreon. In this case only the commissioner has the art.

I... don't think so?
As long as you're crediting the original artist that made it for you, it should be fine. But if you're passing it off as your own art, then it's definitely trouble.

Against the rules? No.
Bad form? No if you have permission to do it, but it's not something I would do myself.