Pony Fantasy VI Remake

So I'm thinking now, if I reuse Vargas's sprite for Dadaluma, that means I can make Dadaluma's sprite something else and reuse it elsewhere, right?

Absolutely, as long as spritesheet imported in FF3usME have the same number of poses of the one you're replacing.

Also, I noticed a few sprites (85, 87, 89) are blank. What are they for?

There's no feature in ZoneDoctor to see where sprite IDs are used. You have to check individual NPC. There's also the NPC data document available on Izmogelmo's site that tell the graphic entity (sprite ID) of each NPC in the game among other stuff. You may try a text search in that document with your sprite ID.

Edit: Not sure but "blank NPCs" may have their graphics loaded in a special way or the sprite/direction combination makes the graphic invisible in ZoneDoctor (possible?). They can also be used for treasure chests purpose. In any way they are not useless, check their event (in event dump) to understand what they do.

Edit: Sorry I thought you meant "blank NPC" in ZoneDoctor. The sprite you are reffering to seems to be unused but you can only insert there three separate 8x8 graphics. A good thing to do would also be to check the NPC data document mentionned above to make sure these sprite IDs are not used by some NPC(s) for a reason.