Potassium pomegranatae would kill all invasive insect larvae or bad snails or nasty bacteria or other parasites?
Yes, it kills everything that isn't a plant.
Just be sure you don't get it on your hands or clothes, it will dye them a dark shade of purple. It isn't toxic to people so don't worry about it if it does get on you (they actually use it in survival kits to purify drinking water, so it is in fact edible).
So I guess I need to get a five gallon as a grow out/quarantine tank?
If you use potassium permanganate then there is no need for a quarantine tank.