Hello,
Cherry barbs are like most fish that like groups on there own they may never nip your betta but if you put them in a group of maybe 6? they feel secure enough to start nipping so a few would be fine but some people have had bad experiences with cherry barbs i have never personally tryed it as i dont like barbs! i had a very bad experience with a group of 13 neon tetras all nipped my betta but tryed the betta with 2 neons and he was fine!
Good Luck,
Luke