I've had quite a bit of experience with tiger barbs and I know I'm not the only one to find they are pretty bad about nipping fins. I would mostly worry about them biting your betta's fins, and they might go after your guppies too. I don't know how big your tank is in gallons - but if you really want tiger barbs, get as big of a school as possible. 6 is the minimum amount, 8 ideal min. The more tigers there are, the less aggression you'll find they exert on other fish. You can always get green tiger barbs, or albino too - they'll school with the normal striped kind, and you can mix it up colors wise that way too. I really enjoy tiger barbs, I think they're very interesting fish. They are always active and fun to watch. Just remember they can get 3 inches or so, so leave room for growing in your tank.
As for the suggestion of tinfoil barbs in an earlier reply - I'd say no way unless your tank is very large. Tinfoil barbs get huge, I often see ones in my lfs that are at least 7 inches long. However, the cherry barb suggestion might be a good one. I have 2 cherries and they're very cute fish. They don't have the typical barb "attitude" and are a lot more peaceful then tigers. However, they don't school very much if you want a school.