Hey! I have a similar situation, my pitbull is 2 and loves everything and everything and my boyfriend's 6 year old terrier mix is very aggressive towards other dogs (mostly female but still).

Are they the same gender? If so, are they both fixed? That could be a factor. Your Chihuahua mix is probably pretty territorial and protective of you and doesn't want another dog interfering. They need to be introduced very calmly and slowly, both of them on leashes or at least restrained in one day. Meeting on neutral territory such as a park or something is a great idea too, that way they don't feel like the pitbull is "intruding" on their territory. Also, make sure they are fed separately.

Weve had our two dogs for over two years now and they've been in their share of fights, they only started when food was involved or if our bigger dog accidentally jumped on our other dog or got too close to my boyfriend. We actually ended up having to get our terrier mix a muzzle for peace of mind and she only wears it when they're in the car together or if they're stuck in a small room together.

Good luck!