It's so funny they school like that, they're like fighter jets or something.
bacteria blooms are good and bad at the same time. They're good in that they indicate the bacteria is thriving...bad in that the population will soon crash and there won't be enough bacteria to process the waste for a few days. same thing happens with rabbits and mosquitoes...and any other organism.
(the process is called "carrying capacity" if you wanted to look it up in your biology books

)
I would do small water changes daily (maybe 10%), but only vacuum the substrate once a week. you want that food for the bacteria, but you also want to keep the levels from spiking. It'll take longer to cycle, but it's safer for the fish.
with your readings, I would suspect you only need to do that for a week. then start extending the interval.
btw: the lfs gave you a halfway truth. The ammonia might be high enough to kill fish (depending on how big they are, large fish are less prone to dying), but the nitrates are FAR from dangerous. My water has 5ppm nitrates in it from the tap!