plenty of people take a 'natural selection' approach to keeping fry. basically if you just leave the females to give birth in the tank and don't do anything to try and save the fry most of them will get ea\ten, a few will survive but hopefully not so many you'll be overrun. the mollies and swordies will probably eat the guppy fry. common livebearers on the whole are not brilliant parents, this is why they have loads of babies and just hope a few will survive as opposed to human's who have 1 at a time and nurture them.