well IMO your choices are
A ) give the barbs away if you want other fish other wise the constant bickering will stress the barbs who are being picked on and sometimes they will die from the stress, not to mention Barbs may turn on your other fish since they are in a small group. (when I first got my Barbs I made the mistake of only getting 3 the dominat one nipped and bulled the other 2 until one of them became so stressed and distraught they died after doing my research I realised the error of my ways and got lots more Tiger Barbs and this solved the problem for me)
or
B ) Buy more Tiger Barbs and wait till you get a bigger tank to get more fish......... if Barbs are in big groups they will leave there other tank mates alone.