All are easy to care for and prolific breeders. Guppies are the smallest of the 3 and Swordtails/Platies about the same size (though sword tail males are smaller). I'd recommend something bigger than a 6.5 gallon though.
mollies are kinda bad, cuz the females are in a constant state of pregnancy. platies breed with a little help from you, like live foods. and try to get an even number of males+females so they each can have their own mate.(but dont get 2 many!) swordtails dont breed as easily. and guppies will breed no matter what after time goes bye.
i agree... this is too small of a tank. even with only a few fancy guppies, the babies will slowly add up. look into something bigger, and keeping some fry. i love it.....i always get to see my new babies being born