rat terrier/ jack russell behavior differences

I was wondering if the techniques here would apply to a rat terrier/ chihuahua mix. I believe the Rat terrier and Jack Russells have similar traits but am not certain. I have a 12 week old rat terrier/ chihuahua mix puppy. He seems very smart. We are having a few issues with house breaking and aggression. He will sometimes growl and snap at people when they try to pick him up or even just pet him.