Barbs? I'd say 10-12. Make sure you have plenty of hiding places or line of sight breaks such as wood or fake plants. Tiger barbs will eat live plants. They are pretty aggressive as it seems you know. I don't think they will act quite like piranha other then the feeding frenzy on fish food, and being mean to each other. I'd say depending on water parameters you may be able to do exodon tetras but it's hit or miss with these in smaller tanks, they also beat on each other. They also need lower ph softer water for good health long term then the mass produced barbs that come from rivers with higher fluctuating water parameters already.