Hi
I believe Cherry barbs are one of the "friendly" barbs however, the issues in a 6-8 gallon are -
a) With a two inch betta it doesn't leave a lot of stocking options, and as Cherry barbs are a shoaling fish you would need at least 5 or 6 minumum.
b) In a smaller environment the betta may not welcome tankmates, as he may consider that the whole tank is his territory.
Have you considered other fish like Harlequin or Copper harlequin Rasboras? The copper ones are smaller and IMO more attractive.
What ever you decide to do, put the other fish in first and the betta last, But i personally wouldn't add anything in with a male betta in a 6 gallon.