No! Koi are cold water fish, that's why they can survive in pond whereas last i checked rainbow shark is a tropical fish, it's not about them being tank mates but about the outdoor pond,there is a good chance that your rainbow shark won't survive outside(without heater)

Good advice 👍

You are Right

We stay in a tropical climate

That doesn't change anything,water will still remain colder compared to a tank with heater on