Welcome! There are a couple of other mixes on the forum, but I don't think they are that cross. Since that is well within standard size for both Havanese AND Shih Tzus, your girl sounds just about right!
Yes, HRI dogs would NEVER be on Petfinder!And sometimes they’re smaller or bigger! My girl is VERY petite at just over 8 lbs (but kind of tall, so I think she’s actually just within standard), and my boy is 17.5 lbs (also tall, at an inch above standard). But it seems like most are in the 10-12 lb range. If you’re interested in rescue, you should fill out an application with HRI (Havanese rescue). They have so much demand that I don’t think any of those dogs make it to petfinder!