Those spells don't pause like Fireball. After selecting Fireball, it shows you the battle screen, it won't finish casting until you tell it where to go, and it lets you cancel the spell. After selecting a spell like Slow Group, it finishes casting immediately without letting you tell it where to go (it already knows where to go) and without letting you cancel the spell. There's no chance to show you the highlight so you can decide if you want to cast it. If you can also make it pause to ask whether or not to finish casting while the player looks at the highlight, your idea will be even better than what I was suggesting :)