I was lucky enough to have a great science teacher in high school who had done some study work in the ceph field, so I managed to avoid the crap that is spread about octos in the pet trade. Even the lfs where I buy all my stuff is somewhat ignorant about octos, to the point of ridiculousness sometimes...all you can do is gently educate them. When you run into a "nutter" (term from kapoc) like the fish store guy you describe, the best thing to do is to run in the opposite direction...they have already made up their minds.
In defense of the lfs, octos are not a high priority or high cash sale item for them, so they don't bother to learn much about octos. They have to answer questions about everything from damsel to coral (which, lets face it, is where most of the $$ is) and have little time or patience for a client who peskers them with queries about a no-profit item...remember, they are in business to make money. I have actually had people register suprise when they find out I mark up my animals for sale...they thought I was selling them for the same price I paid!!!
That said, cruise to some different stores, but maybe give that one another try sometime...the arrogant guy might be gone!