Adding a second dog to our family

Hello,

Several years ago my wife and I adopted our Corgi mix Bentley from a rescue organization. He was our first dog together as a couple. We knew very little about his background, but we were told he had a problem with men and strangers. In his foster home he bonded to the mother and never left her side. So the extent of excluding all the children in the household as well as the father. He may have had a puppy past before them.

It took us about a month for him to get used to me because he instantly latched onto my wife and never left her side. He wasn't a fan of our cat, but he eventually learned to tolerate her. We were thankfully very patient with him and he has formed a great bond with both of us.

He's still really nervous around strangers and other dogs (he hides behind our legs and sometimes barks a little if they're far enough away). He's not really aggressive.

He does very well with our ferrets (who love him) and a kitten who grew up with him in the house.

We'd like to adopt another corgi when we get a bigger place (in about a year). But we're not sure if it's best for him. My wife wants a female.

My question is this: We'd like to avoid disaster. Should we get a puppy? Bentley still barely tolerates the cat who predates him, but likes the kitten who was raised from eight weeks old after he was established. We love rescues, but we're not sure if that'd be better than starting from a fresh slate.

Is there anything we can do to prepare him for our future family member? We don't have any friends with dogs and we're kind of worried about letting him get close to the dogs of strangers, should he get aggressive or find an aggressive "friend."

Any suggestions are welcome! Thanks!