Female guppy killer

[Chico39]

I am new with guppies and have been having problems keeping them. I have a heavily planted 20gal aquarium. I started with 3 females and a male. I added a couple platys since they are also livebearers and supposedly get along. Well i kept finding one after the other guppy hiding at the top in weakened states and some fin damage. They would slowly die one by one. I notice platys chasing relentlessly the guppies. I removed all platys. Bought several more guppies. Same thing keeps happening. I notice an aggressive female fin nipping. I think ill put her with the platys in my 36gal since its aggressive. Well about a week and the platys picked her to death. I have heavy live plants in both tanks for my live bearing fish. I still had a couple more die due to aggression i think. I removed the last female of the color i seen being aggressive. I want to breed these for feeders for my cichlids. But i cant seem to get any female guppies to not kill each other. What should i do?