Thanks for the replies. I put the sunfish back into the river where I caught them. I decided to buy schools of tetras instead.
Oh god. Don't ever release fish from a tank into the wild, even if they were originally from there. In your aquarium, they can be exposed to pathogens, parasites, and infections that they may not encounter in the wild, and even if they don't appear symptomatic, they can spread it to other fish where they are released. You can actually wipe an entire body of water clean of fish life by doing this.
Something similar occurred with frogs - clawed frogs (
Xenopus) were imported from Africa, and they carried an endemic fungal pathogen called chytrid, which they'd built up an immunity to and showed no symptoms. Some people released their pet clawed frogs into the wild when they became bored of them, and within a decade, global frog populations are in grave peril and multiple species have been lost to chytrid infections.
Don't do it! Don't ever release any pets into the wild, even if you collected them yourself. The results can be disastrous.
As to the original question, sunfish are nasty. The best tankmates for them are other sunfish and same-sized cichlids.