Correct. It doesn't matter if a fish is strictly herbivorous, and it doesn't matter what kind of fish it is. If one fish can easily fit in another's, it is possible food. There is always the risk with that combo. A 200 is not enough tank for a hi banded shark, giant gourami, nor a pacu. If you're bringing the koi in every year, I wouldn't add anything. The koi themselves are giants, especially for a 200. If it's a one time deal, then the balas and tinfoil idea would be great, and would look really nice. Silver dollars would even fall into that same grouping. You could pick two of the three and form a couple schools.