All I can think of are otos and if the tank is large enough, bristlenose pleco. I've had puffers before and they don't bother fish. I wouldn't put any invertebrate in there as you've seen, they'll definitely get killed.
The thing about pitbulls are a myth. It's mostly abuse and mistreatment that turns them rotten. And everyone knows that chihuahuas are the yappiest dogs.Pit bulls are considered dangerous, but I've met some pits that were the sweetest dogs ever. I've met Chihuahuas that charged my 180lb+ Dane.