I'm surprised to hear about a 14-day guarantee on the fish. Most LFSs I've been to don't offer longer than a couple of days. Since you do have that, though, you could get your money back on the guppies...but I wouldn't replace them, since your tank was overstocked to begin with. I'd suggest that if you want to keep mollies, platies or guppies that you look at a larger aquarium. Even a 10 gallon is too small for many fish (mollies need more swimming room than that). Otherwise, you may want to return the molly and platies and get fish more suitable for the 5 gallon.
My LFS recommends that guppies aren't suitable for a new (still cycling) tank, and shouldn't be introduced until a tank is well established. It will take a while to fully cycle the tank, and it seems it has barely started in your tank at this point. You'll see ammonia go up first, then nitrite when ammonia starts to drop and then finally you'll see nitrates. The process will tank a few weeks. You do have the option to do a fishless cycle if you prefer. It takes just as long, but it doesn't harm any fish. Check out the "Cycle" sticky at the top of this forum.
