I have a large Setela bag and that can definitely be worn on the shoulder. Yes, it's a larger bag, but because the leather is so soft, the bag slouches and forms to your body. It doesn't look overly large when you're carrying it because of the way the bag drapes. It's also amazingly light weight. It's one of my favorite bags. The leather is different than the Baca. Baca leather is thicker and smooshier. The Setela is a very thin, soft leather with a very light sheen to it.
Also, to answer the question about the zippers... I haven't heard much about problems with zippers, but there have been lots of reports here (on TPF) about loose stitches. One thing I do know is that Gustto has one of the best customer service departments when it comes to handbags. They stand by their products, and will likely fix whatever needs fixing on one of their bags. If you bought a bag & the zipper broke, I'm sure if you contacted Gustto, they would be able to fix it for you.