It is possible to do a fishless cycle with TSS but I would advise against it unless you constantly check your water params, otherwise your kinda shooting yourself in the foot since the bacteria will be waiting for you to deliver an ammonia source rather than having a fish that's constantly producing it. It would be nice to see some data on a fishless TSS cycle though to get an idea of what works and what doesn't.
Another recommendation....general good practice is to use a quarantine tank for new introductions, even when you trust your LFS and in this case I assume there's still not one found that has a "track record" you can count on. The 10 gallon is ideal as a quarantine tank for small fish. Also, if buying platies, decide whether a mass number of babies will be feasible. If not, buy platies of the same sex.
Sorry to hear about your experience. I went a little less whole-hog, but had my first 2-3 fish die horribly in my brand new tank after keeping a couple alive in a smaller one for a few years, then found this place ... and have learned a lot in a short time.
My son lost interest in having a running, yet empty tank for about 6 weeks. He thought the tests and color matching were fun for about 2 weeks. And for most of that time, I didn't think I had a single bacterium in the place.
The trick for me was borrowing an absolutely filthy old filter from a friend of mine. Suddenly the ammonia disappeared and the nitrite not long after, and my undergravel pulled lots of bacteria into the gravel.
Now, my son and I spend some time checking out the fish each day and feeding them together. Our favorite game is "find the sucker-fish" (the Bristlenose Pleco).