Tiger Barbs should be in a min 30g. They get size on them and they need room to dodge aggression. They are an active swimmer.
I would return the TBs and look at Nano fish or a single Betta.
Jellybean Rasboras would work. Just them. Not other fish. Not with TBs
Or buy a 30-55g and look at stocking a semi aggressive tank.
So my answer is ZERO. Zero Tiger Barbs in a 6g. It's not an appropriate home. I don't care what the store said. Note what LiveAquaria says
Sorry I know this isn't what you wanted to hear. Tiny tanks under 20g are tricky to stock appropriately.