Is my Chihuahuas aggression behavior or psychological?

I rescued two male chihuahuas about 4 months ago. Both were malnutrition when rescued. Max is 2 years old and Buster is 5 years old. I had both dogs neutered and brought up to date on all immunizations, heartworm and flea medicine. Every since we took them in they have been antisocial and somewhat aggressive. Max is aggressive. He is more aggressive then Buster and I think Buster’s aggressiveness is feeding off of Max’s behavior. When my husband tries to pick Max up off my lap Max snaps at him. Max goes in his cage willingly on the command “House” but if my husband tries to close the door Max becomes aggressive. He is fine with me closing the door. Both dogs are aggressive and territorial with each other and with our collie Logan when eating. They like Logan and most of the time all three dogs get along well with each other. We can pick up and put down their food bowls without any problems. We have a fenced in yard and a doggy door in our sewing room. Max and Buster have to be secured in the sewing room or their cages when any of our kids and grandkids visit because we fear that Max or Buster will bite someone. I have discussed the issue with our vet and we started Max on Fluoxetine hoping that it might help calm him down. It is too soon to determine if the medication is making a difference. We have no idea of what they have gone through in their past. Max does have something wrong with the muscle in his left back leg like he might have suffered an injury but he is not in any pain and gets around well. We need to figure out if the aggression is a behavior problem and if there is something we can do to help him. We also discussed the fact that Max might need to be put down. I need advice/recommendations on what I can do to help Max and Buster because they have been through so much and have otherwise come a long way. Thanks!