There are no rules to break unless you are entering images into a competition, which may have rules on processing at least. Doing it for yourself, do what you want. Want three stones in a foreground of a panoramic long exposure water scene in the golden hour with a different sky! Go for it. If it is an image for yourself, which all images should me imho, do whatever makes you happy.
I go through phases of doing different things. I'm planning on some long exposure traffic trails on my next trip because I will have views over a city. Cliched! I don't care.
Like most people I have my own limits of what I'm willing to do when it comes to processing an image, even if it is just me seeing the images. I got the changing sky thing out of my system very early on, so it is not something I would do, but if anyone else wishes to do it, go ahead.
But do it well or expect some 'feedback' from people you show your images to.
That said, that doesn't mean it is nice to be fooled and disappointed by an altered image, as I was went to Manarola in Italy, only to realise that an image I had seen online with a nice Milky Way behind the town was not possible without replacing the sky, and not a sky from the same place either because of the light pollution.
Luckily the town was nice enough with a normal sky.
There is also an image of the light streaming through the whole in the roof of the Pantheon in Rome that is sold on fridge magnets, postcards and posters that I had seen in tourist shops on my first visit that I wanted to try and do myself, only to be told by a guide there that the light in the Pantheon never comes down vertically as it is not on the equator of course. Doh!
It makes a nice fridge magnet though.
The building itself made up for that minor disappointment.