Actual replaceable monsters?

[darkskyx]

This may look like dumb questions but I didn't see it in the forum. There are many questions about dummied human units but none about dummied monsters that you can easily change without losing content.

I started to look sprites into Shishi and I saw 2 dragons. I watched the sprites and the palettes and everything is the same. Why is it like this?

I also saw that Bremondt uses another dragon but the sprite is a little different. And also I think it is coded as human because he uses skills. So maybe he is not a good option to be changed.

Now I ask myself... Can we use another unit (like an NPC) to be used as a monster or new boss? There is a list of hardcoded units? Those are the questions I ask myself. I prefer to ask before testing like a madman. I'm using the PSP version, btw.

Oh, and can we replace sprites number 00, 9B, 9C, 9D, 9E? Are those sprites really used for anything? Because it just looks like a black screen. And I don't mean as a unit you can use, since you can replace another usable unit. I mean if you could use those spaces as a way to add unobtainable new monsters/bosses there.
Edit: Nevermind... I found this comment on the FFTPatcher post: "These are blank and unusable with a size of 0 unless ISO is expanded."
Edit2: I'll try to replace Holy Dragon that is using an unused Dragon, by also swapping it with another monster so it could be used in your party.