the combo can work ok - the general approach with tiger barbs is to have 10 or 12 or so - this generally keeps them busy with each other, confining aggression within the school. But there's no accounting for individual fish. You may find they settle down as time progresses. Remember, everyone in the tank is just getting used to it and each other.
12 Tigers is way to much in a 10 gallon I think. I would give it some time, and keep up with the testing and water changing.