You CAN overdose ANYTHING that is added to a saltwater tank.
Ask your LFS lady what would happen if you dumped in 20 big bottles of this stuff. I'm sorry to say, but I think she gave you horrible advise.
Dumping an entire bottle .. the biggest you can find .. of anything into a tank is a DUMB thing to recommend to anyone.
You asked in your title ... so I'm giving my opinion
If these bacteria in a bottle are indeed still alive ... then adding them to a tank can cause problems.
They are alive .. they consume oxygen ... fish need oxygen too.
It's like forcing a bacterial bloom - by pouring in these bacteria.
Not necessary as far as I'm concerned.
I don't think it's wise to upset a marine tank by pouring lots of "anything" into it ... it disrupts the balance that we try so hard to maintain.
Also ...... I agree with StacyT.
In theory .... quickly adding a bunch of aerobic bacteria in a bottle ( Cycle ) would do nothing to lower nitrates.
They may lower/convert ammonia to nitrite, or nitrite to nitrate ... but they would not convert nitrate to nitrogen gas.
I think your LFS person should be avoided in the future.
Poor advise for sure - IMO.
Water changes are in order - as StacyT suggested, and possibly increasing in tank water circulation.
Eventually the excess bacteria will die off - as there are only so many required - and so many that can live off of the available food sources ( ammonia and nitrite being just two of them ).
