Yes, Guppies (Poecilia Reticulata) can be cannibalistic. The best method that I have found is to provide plenty of floating plants for the young to hid in. I feel that breeding nets and other enclosures stress the female. For instance, I have two tanks with at least 45 guppies (try to count them,..). This originally started as one tank with three guppies which I set up just to give my cats something to watch while I was out. I knew their propensity to breed so I threw in some floating plants and now, less than year later, I have had to set up a second tank to ease overcrowding.