I picked up three platys earlier this week at petsmart and they told me two were female and one was male. I'm sure the two red ones I got are male and female, but i think the sunburst might be male instead of female. I'm not sure, and I want to keep a low ratio of males to females.
Seems you figured how to do so. I saw your post which fluttermoth pretty much answered. I'll answer you here anyway.
Fry need:
Hiding spots
Floating plants
Daily water changes
Normal food crushed up so it can fit in the frys mouth.
That's all. Not much but it will keep the fry healthy.
~Karen