Welcome tot he Forum!
Ick meds (white spots) are administered at half does if dealing with Tetra, as they're real sensitive fish, the full dose was simply too much for them. Platy
not only digest a lot, they also have a LOT baby's which can quickly outcrowd your tank, but i'm hoping in your case the others will eat them for snacks.
What size tank are you having there?
If you have ammonia present now, for one I'd suggest a good size water exchange. Two that means you tank either wasn't really cycled when you thought it was and/ or it is re-cycling again now, so several larger w/c may be ahead of you.
Ick is present everywhere, if fish are stressed (with transport home, with NO peaks or Ammonia peaks or by tank mates) this then can easily break out. A well maintained tank is less to be affected.