My platys are coloured at birth, ditto the swordtails.
LFS will take them when THEY think they can sell them,
at the size you have available.
I agree with separating from the parents and go another step, separate the bigger ones when about a month old.
Maximum water chage for maximum growth.
I read some where,
that every time you go past the tank, left to right, FEED them,
every time you go right to left past the tank, CHANGE the water.
On no account let the water become polluted with left over food.
This means ammonia, which means baby wipe-out.
Under-gravel filters are okay or a sponge filter, they wont suck up the babies.
You wont make big $$'s from them, so don't get your hopes too high.
Alan