I'm attracted to the concept. I might be interested to see what you've got if you get to the point of writing something down. Of course, it wouldn't be wrong to just have these character concepts if you came up with all the details of the characters but didn't have much of an idea of what to do with them.
I also have a few thoughts. Firstly, a bit minor but why does the ruby control wind rather than fire like it seems to me it should? Switch ruby's and zircon's powers. unless you're trying to go anywhere against people's expectations.
Garnet and pearl seem to me like the odd ones out. If you were looking for more elements, poison is an uncommon choice for elemental powers. Might give that to amethyst but now you have to reassign darkness and that makes more complications for how you match powers and stones. Tapping into pokemon, psychic or some sort of mental powers is another option. Which itself doesn't seem to have any presence in the mge's texts.
I also thought that 2 stones (you could just use garnet and pearl) might augment the owner's physical and magical abilities. The former could give them larger muscles, increased strength or stamina, greater combat prowess, etc. Give it to a cursed sword, or an ogre/minotaur/jinko or other strong monsters.
The magic enhancing stone could increase their aptitude for learning magic, make their spells stronger, give them more energy for magic. Give it to a dark mage, baphomet or witch, demon, I think. Gazer. There are several magically talented monsters.
As they are, your choices of monsters to receive the stones are pretty solid. Lastly, you might find it helpful to check out this video by the channel, Hello Future Me, on writing elemental magic systems.
A few more questions that could help you make some decisions on these characters (some of which I imagine you may have already considered).
Are all of these stone holders going to work together, or are they going to become adversaries? Maybe one of them wants to get more power by owning multiple stones.
Did they all receive their stone directly from the incubus or could they have inherited one from their family?
Are all of them born monsters? It could be interesting if one of them was a human and ended up being monsterized before receiving her stone.
Other obvious details like names, where they came from, one thing that could help to distinguish each of them is how they hold their stone. Some could hide it on their person, some might wear it proudly on jewelry like rings, necklaces, earrings etc. Engrave it in a weapon, the automaton could engrave it on her body.
Lastly, what are their ambitions for these stones? This of course relates to what the stones do. Are they just very powerful artifacts or could the be a lord of rings situation where they were all created by someone for a purpose? I could imagine some of the owners, like the wight or dragon, would become very public and renown for their stone, attracting many people trying to get a look at the stone, assassins trying to steal it, suitors trying for their favor. Maybe others are entirely unknown, and people have no idea other stone owners exist, or they're searching for those stone owners, who are either trying to hide or don't realize the importance of their stone. This could tie into the other idea I had earlier, where maybe one of the owners is a human who ends up becoming a monster and receives the stone in secret.
I hope some of this stuff is useful in developing your idea, but in the meantime I've actually made myself pretty interested in this concept. How would you feel if I took a crack at writing something related to these characters? I have no idea exactly how I would write it, or when I would try to write it, but I would credit you with the idea. I can also link this post if that's helpful.