Guppies, like lots of fish are the subject of generalizations.
I have trio of Bluegrass that have reproduced...............like guppies.
They didn't eat their offspring, and don't chase each other much either. One of the females is now WELL beyond 30 days and looks grotesquely huge. All in a 10G with just a clump of Java moss in it.
I also have a colony of red-tail platinums that are just non-stop motion in a 15G.
There's plenty of chasing and harassing for sure. I've had them three weeks and only one drop with none of the others looking close. Another "violation" of the 30 day rule. But I haven't had any losses and nobody looks tattered or even shy. I caught the one drop in a Marina Box as I didn't think I would get lucky enough that these guys would leave the newborns alone too. I'll take the chance with the next drop though and just see what happens.
Just a hunch, but foods offered may play a role. All of these get a rotation of mixed flakes, Live BBS, frozen daphnia, and chopped bloodworms. Unlike my other tanks, they also get plenty at each feeding.
I have a friend who raises largely half-black pastels and his fish leave their offspring alone as well, and I gotta say his tanks are really packed. Aggression is just about non-existent too.
Adding a couple more females really can't hurt. But as noted, they can be tough on each other too.
Yet another contradiction: as I separate the juveniles out the of Bluegrass trio into male and female grow-out 10G's, the gaggle of females couldn't be bothered with each other, and to a large extent neither can the males I've segregated.
In the end it's about taking some chances to see how things work out. Pairs are displaying aggression issues, so spreading out that aggression sounds like a logical option. If you're offering flakes only, some frozen in the mix may help, but I offer no science on this one.
"Why can't my wife see all of this stuff as an investment?"