First of all, Tiger-barbs will be an OKAY fish to cycle your tank (minimum 1 month), but once it's done cycling (which won't happen unless you have fish or something else producing Ammonia in the tank) i'd return them as they do best in no less than 4 in a 20 gal. Barbs are a little bit of a "nippy" fish and will chase slower/smaller fish around and nip at their fins. They do best with other barbs. So, when your tank is DONE CYCLING return the barbs, unless you want a barb only tank. If not, get a different fish.
Although I have had luck with faster fish such as tetras. I have a Electric Blue Lobster with my Tigers, they're fast enough to get away.
But if you want more fish, stick to small ones, like guppys. Or tetras.