I would just research the breed all breeds have their pros and cons and all breeds have aggressive dogs. There are definitely sweet chihuahuas out there. They can be territorial over their favorite person. They can also be very jealous breed. Aggression is all about training and socialization. I work at a shelter so I have seen bad and good in every breed there is no one breed that is truly less aggressive than another all problems are owner created and it's sad. Me personally I don't like jealous possessive behavior so chihuahuas aren't for me unless they are the exception to the rule. I do think people get dogs without doing breed behavior research first and they end up getting returned for what to their breed is normal behavior. I have probably been bit by every breed imaginable so I would choose based on dogs personality. I love pitbulls one of my favorite breeds but they are too strong for me to even walk so they are out of the question for me even though they are the sweetest dogs personality wise. I used to want huskies but we get a lot in because they are destructive and I find them hard to work on so even though I think they are beautiful dogs, I no longer want one. I would spend some one on one time with dog before adopting. I just hate to see return dogs simply because people don't know what they are getting into