Platies and tetras like to be in groups, the more the merrier. And they like to have room to swim around. I'd definitely recommend getting a larger tank and adding a few more. You could get a solo type fish for your six gallon, something like a
betta.
Just as an fyi, clerks at pet stores like Petco, Petsmart, etc., very rarely have any clue what they are talking about so take any advice they give with a grain of salt. If he'd had a clue, you wouldn't currently have any fish as your tank would still be cycling (unless you got some BioSpira which Petco doesn't seem to carry).