better late than never! so it's good to start socializing mia with other dogs and people now. maybe start with a more quiet or personal setting than petsmart. playdates, small dog playgroups, or even doggy daycare are a great way to start socialization.
bunjee was a bit timid around other dogs at first too. he's still a bit of a breed snob, he plays betters with other yorkies than other breeds, but he's coming around.