And what you heard about breeding traps is correct. Don't use them. If you want to attempt to raise the fry just put her in a tank by herself with lots of plant cover for the fry to hide in.
Liquefy is not a very good first food and is more designed for egg-layer fry which are smaller than livebearer fry. Your fry will be able to handle finely powdered flake right away. Hikari makes a decent one, Hikari First Bites, it's powder-fine. But if you want them to grow to their full potential and not be stunted get some frozen baby brine shrimp and/or frozen cyclops. Both are generally available these days in pet stores. Do you have a tank in which the fry can grow out?
Going back to you initial question, assuming that a female has been in the presence of a male at some point, female platies are pretty much always pregnant. They can have several broods from just one mating. Something to think about when you consider whether or not to raise the fry.
