Rasbora nippy?

I introduced 5 rasboras (hets) into a 4 gallon filtered tank with my
betta, after a few hours of making the rasboras nervous, everyone settled
down and appeared to live in peace.

A few days have gone by and my betta now has fin rot. When I went to the
pet store (mom & pop) to get some treatment, the guy working there is
convinced that the rasboras probably started the fin rot by nipping at the
betta's fins. I've never seen this behavior and I like having them in the
tank but I've seperated them for the time being while the betta heals.

I bought the rasboras at a different store but this guy offered to trade
them for a cory cat, which he says will be a more harmonious companion for
the betta.

So, what do you think? Are rasboras nippy fish or is it more likely that
they introduced the fin rot (so far they don't seem affected) and the
betta was just more susceptible?

--
Mac Cool