No worries - lots of people find their pups on Craigslist. You're lucky that it wasn't a scam. He seems like such a great fit with your family.
Don't worry about whether he's Biewer or Biro. It is really only important to a breeder.
I'm sure he'll bring you a whole bunch of fun and love. Even tho he's small, he'll still be pretty feisty (major terrier trait.) I bet he'll take that boxer of yours to task too every once in awhile!
Enjoy him!!
Best wishes,
Tracey