Welcome to the forum Pincadillac!
Wow, 17 platies huh? Although I feel it is always bad advice, your LFS suggesting 17 fish would fall into my definition of overkill and then some.
A good place to start might be to check out some of the information contained in FTF's
Articles section; which you can access by using that link, the link located in the upper left hand corner of this, or any, forum page, by using the link located in my signature below, or by clicking
HERE, which will take you directly to the article on cycling, which includes information on fish less cycling as well.
It might also help if you could share the results from your most recent set(s) of water testing. This way we could help you understand what they mean and how to help your fish deal with any less-than-ideal water quality problems. It might also help if you could share your current maintenance routine (i.e. frequency/amount of water changes, feeding schedule/amounts, etc...)
Personally, I might try returning these fish to the store and move towards a fish less cycling method since this would help ensure that the platies do not suffer or die from your current situation. Otherwise, depending on what your water tests are telling you, you might look into adding a bacterial product like Bio-Spira so you can hopefully increase the size of your beneficial bacteria colony faster. In fact, you may also be able to purchase some sand or substrate directly from your LFS, place it in your tank (perhaps in a filter bag or clean, unscented nylon stocking so you can take it out later on if it doesn't match) since this would also help bring beneficial bacteria to your tank. Of course, you can use any substrate and/or filter media from an already established (i.e. cycled) aquarium if you have one or have a friend who has one.
Knowing more about your tank and your water test results will help us give you more accurate info and/or ideas which pertain more closely to your unique situation.