Ho man! All you need to do is get a natives permit and go collecting! You should try and locate a legal dealer for different darters. That'd be cool, though it would depend on where in California you live. If it's cooler some sculpin would be awsome. The type of sunfish that would survive, or at least thrive would be cyanellus, and they'd all move to the pool. Otherwise, yes, channels would work, as would flatheads. The bullheads aren't much for moving water, but you could do madtoms, such as N. flavus. They get big enough to see and that'd be quite cool. If you can provide food, all sorts of suckers would work. I'm not familiar with the cyprinids of California, but a large school of Notropis rubellus or another Notropis would really look nice. I'd look into catails, elodea, and other aquatic plants for filtration. The pool would work nicely for that. The possibilities...