Chihuahua's are really cute dogs and sometimes it's really hard to imagine that they are aggressive. I read somewhere that they do not get along with other breeds and they are not suitable if you have small kids because they have the tendency to bite when they are scared.
I agree that with proper training, I'm sure you'll have fun with your dog.
By the way I found this article about a Chihuahua protecting a 3 year old girl from a mountain lion.
Chihuahuas save 3-year-old girl from mountain lion - KSBY 6 Action News San Luis Obispo, Santa Maria, Santa Barbara
Just thought you might want to read it.