I'm a little confused with your spawning method. Why are you separating out the guppies from the main tank to spawn? You know if you just leave them in the main tank, the male will eventually spawn with either or both females.
If you feel the platy is near giving birth, you can use a breeder net. I don't use nets. I just have java moss in the main tank, the fish give birth, the fry go into the java moss until they're old enough to come out.
Still, my biggest overall concern is that these fish are now in an uncycled environment. They could die from shock as I presume you did not acclimate them.