If you start adding livestock when you should be cycling..you're gonna have a bad time.. (Insert southpark skier picture here)
But really, it's some of the best advice that nearly no one listens to..I lost $100s in fish starting out. Money aside, I felt terrible for all the little fishies I killed because I chose not to listen..
Unfortunately a large percentage of LFS will do whatever it takes to make a sale RIGHT NOW instead of actually helping you, and potentially keeping your business long term.
I'm not saying this is the case here as I don't know your LFS, but its sounds like iffy advice from them for sure.