I'm pretty new to this whole fish keeping business but I can share my own personal experience with guppies, platies and mollies. In the beginning we bought 3 platies (by chance we got one male and two females) and asked for 3 male guppies (didn't want babies!). What a mistake! The guppies were always after each other. One in particular became very agressive and kept attacking my little black guppy - he died. He then began on the other which also died (I believe from stress). At this point my platies were also a little stressed. One of the females died too (not quite sure why) but at that point the remaining female hid for 3 days and the male just sat in a corner. It was pretty sad to see. I went out and bought 3 more female platties and what a difference!!!! The orginal girl came out of hiding and all 5 have been happy, active and playful ever since. I sure do wish my lfs had told me to have a larger community of platies.
At the same time we bought the 3 females we purchased a male and female mollie but the same thing happened. He was always after her and she died. So we bought 3 female mollies and all is well in that community too!
The remaining guppy became a holy terror. He attacked the mollies constantly, causing one of the females to begin to show huge signs of stress. As I'm still in the process of cycling I couldn't purchase any more fish (should have bought 2-3 females for him to, ummm, play with) so I just removed him from the tank.
Now, I have a very active, stressfree tank which is really fun to watch!
Morale of the story? I think the rule of 2-3 females for every males is a very wise one! Otherwise they will become agressive and reak havoc on the tank. Pregnancies will occur but you then have the choice of a fry tank and returning the babies to the store for credit, or, letting nature take its course. The other fish will eat the babies...... it's what's been happpening in nature forever.
Good luck! Enjoy your tank. I love my platies and mollies - they're great fish to start with. HTH
Val