If you have kept guppies you know they do much better in larger groups. This is especially true if you have multiple males. Having a male guppy and just one female is really a bad idea. Doing well in groups is actually something guppies have evolved to be good at. First off, guppies have a natural tendency to spar and when the group is large this is spread out so that no one individual gets too much of it. Secondly, males like to mate, a lot, and often. A male will mate a female to the point of exhaustion and death. So a male needs a harem of females for their own health and if there is more than one male they need a group for the males’ health.
When keeping guppies I’ve found them to be better off with no other kinds of fish (with the exception of cory) in the tank with them. I’m not saying a guppy can’t go in a tank with other fish, but if you want really healthy guppy and you want them to have lots of fry I’ve found a specialized tank to be the best bet. If you don’t care what the offspring look like but just want little ones, then you can mix up the colors. Guppy aren’t particular and will mate with whomever there is to mate with. So if you want a tank that looks like a bubble gum machine with all different colors, go for it.
Keeping fish in a proper tank, with proper tankmates, and in proper numbers is the job of the hobbyist. To learn those ratios and pairings is part of the fun. Research done on guppies has determined what we know, they are able to determine the size of a group and know if it’s a good-sized one. A guppy from one group, perhaps that might be too small of a group can see another group, determine its size, and decide to join as there is a benefit in that. When nature has bred something into an animal, when evolution has determined how this thing should best survive, it behooves us as keepers to recognize and honor that. Some people think of guppies as disposable fish, and that’s just not true. They are deeply rewarding and their ability to livebear their young means they are one of the most interesting fish you can keep.
Add to that the fact that guppies (like a few other species) can be acclimated to full marine and you can keep them in a sump where they can produce fry that end up in the main tank as live food for your reef. Or, if you like, you can put them and molly in the actual display. It’s an amazing little fish that shouldn’t be thought less of.
Guppies can’t count exact numbers like people and a few animals (e.g. African grey parrots) can. But they do possess the ability to discern “more” vs “less,” and some guppies are better at this skill than others.