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Is the 25 gallon the permanent home for the shark? 25 gallons is a bit on the small side for an adult; they need about 50 gallons when full grown due to their territorial nature. Keeping them in smaller tanks leads them to attack their tankmates when kept in a community. They also get about 6" full grown, so your ghost shrimp could turn into snacks later... You're fine for now, but just some things to be aware of for later...
Yep, they start off community-friendly and passive. My shark was a total love until he grew older and hit around the 4" mark. At that point the teenage/adult personality started coming in and he wouldn't tolerate anyone in what he considered his territory, which can be a fair range. Even in my 75 gallon tank I sometimes have to rearrange to reset his territory whenever he gets too bossy.
I agree, the rainbow will probably turn into a jerk, eat all the shrimp, and become a total downer on the tank. But hey, might as well see what happens, maybe it a midget rainbow that is a sweetheart! Haha