If you check our profile--click the shaded name Rosy Barb
--you will see it mentions reports of fish being nippy when kept singly but less so in groups. Jackiebabie's experience is not surprising.
Shoaling fish are that way by nature, and always with reason. One reason can be interaction between fish in the shoal, and this can take many forms. Barb by their nature are more active and feisty than say danio, or rasbora certainly which are extremely peaceful. So with such fish like barb, more is always better than less. Tiger Barb
for instance have proven track records of being less trouble in groups of 8 or more.
When a single fish or two is added to an existing group which is already too small for the species, trouble is often severe, as you've noticed. This goes for many shoaling fish, be it angels, Discus
, some tetra, etc. The "pecking order" in the existing group is threatened by the "intruder." If possible, always buy all the fish in the group together. If you need to later add more to replace or just expand, add 3 or more together. Less trouble will ensue.
The Fire Barb is not yet in our profiles, so here's a link to an article by Matt Clarke in PFK: Fire barb, Barbus fasciolatus | Features | Practical Fishkeeping
Matt doesn't give numbers for the group, but I would suggest 7 or more, acquired together.