I have 6 green tiger barbs, 9 Danios and 1 ruby shark and all are new - 2 months from the store. I was advised to buy no less than 6 and they are fine, if anything they are being hassled by the shark as they are very territorial and don't like the barbs around the bottom of the tank. However, none of my fish have long fins and this is what you have watch out for! We were told that gtbarbs and angel fish were a big no no - even long finned danios were too! I haven't been doing this very long but after much research I am under the impression that they are much better in groups so they have their own kind to hierachy! Like I said we have 6 and I don't know if it helps but the shark does give them regular reminders of who is in charge even though he doesn't seem at all bothered by the danios and the tank is only 27 litres. We are currently looking for a bigger tank as they will all grow but they still need to have swimming space so make sure that they have enough room too otherwise the emphasis will be on the other fish!
Hope this helps!