Thank you for your input.
I'm aware of FFXED and how it allows you to use skills presumably through some overflow bug in the game if you set item id's beyond what is normally obtainable within the game itself.
I cannot find said videos at this time and it's been a while since I saw them so maybe this was indeed what was used there, though I believe what was actually done was modifying some black magic spell and not done through an item.
Regardless, this Final Fantasy Wikia page lists a bunch of enemy skills with correct parameters:
http://finalfantasy.wikia.com/wiki/List_of_Final_Fantasy_X_Enemy_Abilities.
Granted most of that is obtainable from just observing their effects in the game, but the power and accuracy values are interesting and do indeed match the enemy skills that reside in the game's data so they have probably been retrieved from there one way or another. The following quote from that page also suggests this:
Abilities listed in italics are not named in-game, but are named as such in the coding.
Upon inspection, it appears that both shared spells as you call them, and enemy-specific abilities are stored in the very same 96-byte structure (this is in memory upon running by the way so the way it is stored in the game files may be different). While roughly interchangeable there still appears to be something else influencing the skills however; for example I replaced all the bytes of the enemy skill "Meteor" with a byte-for-byte copy of the shared spell "Flare" and killed a Behemot King. He casts this skill as a final attack, but he still uses his "final attack" casting animation instead of his normal spell casting animation (which is normally used for Flare, which this monster can also use normally). Furthermore, Flare is a single-target spell (as denoted, and changeable, from its skill data) but Meteor hits all targets, as does Flare when used as his final attack. The camera animation likewise seems different to that used when casting Flare normally, but it doesn't make the camera look to the heavens as is done for the Meteor animation. As such there does indeed seem to be some other component at play here...
If I understood well, the spells that are normally used by enemies on the party has always a camera centered on the party when casting (even when it's cast on enemies through hacking)? Or is it the other way?
Not quite, I have recorded a video showcasing the results here:
https://youtu.be/RwUtX31rvlcThere definately is quite a lot of camera animation in that spell when used "properly" by the behemoth as can be seen at the end of the video, but as you suggest it doesn't do more than show the character casting it and then the camera movement seems to stop there. It might possibly be that the character animation is supposed to tell the camera what to do next, and since there is no such data for the used character animation, it stays where it is until after the spell animation is done. I am not sure how much sense such a system would make though; just imagine all kinds of spells that use the default casting character animation but still moves the camera differently based on the spell. The functionality to do it differently certainly is in there, so then one can wonder why they would bake it into the caster animation in certain cases?
That's for FFIX, though. But I think IX and X are quiet close in terms of data structures. I've already seen some of them in IX and told myself "Ah ! They used the same system for FFX.".
Indeed, it's quite cool how the older games tend to be relatively similar with regards to certain data structures.