I suppose there's an advantage of adding new battlebacks since Lawmage Academy runs at 1280x720 whereas the battlebacks are 1000x740 resulting in them being slightly stretched. I do notice this with my game (mostly the title and game over) since it runs at 1280x720 and the default graphics are 816x624 resulting in them being stretched. It seems interesting that the FSM tilesets didn't come with matching battlebacks like a dark forest, water cave, or even the dungeon/prison.
Since you would have more than 4 characters (Syle + 3 friends), I wonder if training the party would be an issue where only the active party gains EXP while the reserve members do not resulting in characters (for example, Ein or Alyssa) being weaker than the other characters. I know some games compensate by scaling the party's level or allowing all characters earn EXP which RPG Maker MV has as an option so the unused characters wouldn't be weaker compared to the rest of the party.