Not to rain on the parade anymore, but those barbs will likely pick on most anything that you add to your small tank from here on out. They can get pretty territorial, especially in a small tank with limited hiding spaces for other fish. I don't know how attached you are to them, but I would agree with cinder. Upgrade (which you still might need to be careful) or return them and get some nicer schooling fish. I have a 10gal fresh (along with a 55) but in that tank I have kept for a while now:
1 red beta
4 zebra danios
8 neon teras
2 sunset platties
I do have alot of filtration on this tank, but there are plenty of options, activity and plenty of color to choose from for what you have.