What about redtail sharks they’re very common fry a reasonable size and would be pretty when fill grown 6”I'd think Bullhead catfish may be your best choice based on adult size (under 18"), availability and hardiness. You could do Channel cats, but they tend to get large. If it was me, I would go looking for the Yellow Bullhead.
Maybe do some research on cold water loaches....that could be an interesting option.