No you do constant testing and water changes. I posted two articles in my post, you really need to read them, they're not that long I promise
and yea pet stores are notorious for telling you something is ok when it's definitely not ok. The tetras need a group of at least 5 to be happy, and the live bearers need a1:2 male female ratio to avoid aggression. The tank is just too small for all those fish basically. I know it sucks to be told by an employee that everything is fine when it isn't, it happened to me when I first started out :/
You can go ahead and test again now, then post results.
Products that claim to cycle your tank are frauds in my personal opinion and most people on here also say to avoid them.