Platys and swordtails can interbreed. Both are of genus Xiphophorus
Mollies and Guppies are both of genus Poecilia,
however from what I know, interbreedings are kind of rare and the offspring are mostly infertile.
I've heard about the thiaminase but only from feeder Goldfish
. (sorry 1077, the link you provided won't load) I haven't heard that it was present in feeder guppies/livebearers too. Breeding our own feeders is alright, especially if you take care of them well and not overcrowd them. Feeders should remain as treats only and not the main diet. It's nice to see the natural hunting behaviour once in a while.
If you're going to feed only once a month, then 2 females and 1 male would be enough, IMO. 1 female and 1 male would be possible, but a 1 is to 1 ratio leads to the male harassing and stressing out the female too much, which could lead to disease.
Edit: I googled a bit and found out that most freshwater fish do indeed contain thiaminase. However, consensus holds that feeding not more than once a week is not harmful, especially if the main diet is nutritious enough. So if it's a monthly treat, nothing to worry about except for disease, which is easily preventable given proper water quality management.