It really isn't to early to sex them, if you use a magnifying glass and a flash light you will be able to see a black dot just above their anal fin, that indicates that it is female. However, if this feels like it's to much to do, wait about 3 weeks for the anal fin to become pointed but watch closely to the others, I'm still catching "new" males in my three month old fry that I had previously believed were female. You'll want to catch their gender as soon as possible. They are not called the millions fish for nothing. I learned that the hard way, I now have over 40 guppies and counting.
What I highly suggest is keeping the males and females separate. Only put them together for desired breeding. So far my largest batch to be born was some where between 20 and 26. you may put the babies back in with they adults once they are considered big enough to not be thought of as food.
Just a small tip, if you feed them dried baby shrimp I highly recommend crushing all of the shrimp in a zip lock bag. Make sure that they are fine pieces, you can do this by rolling a round object over the bag. After that is done put the shrimp back into it's container. Instead of reaching in to pick their food up like one would do with flake or pellet food, it's better to gently tap the shrimp out of the container and into the tank. Believe me, the crushed shrimp will stick to your fingers like Velcro now.
