All livebearers are considered omnivore species, which need a variety of proteins and plant matter in their staple diet. In the wild they would eat insects, insect larvae, copepods, shrimp, algae, plant leaves, etc. They really are opportunistic eaters.
In an aquarium I like to offer different foods to keep the overall diet varied. There is no single food that can provide everything needed, not even a prepared food sold for feeding in the home aquarium can provide everything needed for growth and overall health.
I like to have proteins in the range of 40-50% generally, and prefer quality proteins, such as a variety of shrimp, insects and even fish.
Good protein sources include whole meals, such as whole salmon meal or even fish hydrolysate, which basically is fish meal before all the liquid has been removed. Insect meal is another great protein source that is found in some higher quality foods as well.
As for plant matter, there are many, many options. Along with spirulina, kelp and veggies in processed foods I like to add fresh, raw veggies and even occasional fruits. I try to get organic, but not always possible. Cut a small bit and stab it with a fork to weight it down in the tank. Cucumber, zucchini, mushroom, green beans, broccoli, etc. Easy to get into the habit of setting a small amount aside for the fish before cooking to prepare my own meals. My fish go crazy over strawberry and tangerine slices, but they can quickly pollute the tank so those are offered the day before a water change.
Good treats include live (so long as you could trust the source), frozen or even freeze dried, but rehydrate first to avoid digestive issues.
Everyone has their own idea on what the the best diet may be. Personally I believe if you offer a variety of foods, including some fresh veggies as well as a few fruits along with a varied protein diet you should be able to provide the correct balance.
In my tanks I feed Guppy Color Feast (a granule very high in protein at 52%), Guppy Flake (a mix of veggie and meat based flakes, 40% protein), and if needing a color boost I add Guppy Color Flake (47% protein focusing on shrimp sources to boost oranges and reds). Then, for veggie content I occasionally add fresh, raw veggies. Just a small bit which I can stab with a fork. No cooking, no blanching as I want to retain the nutrients. I also on occasion feed live Baby Brine Shrimp as well as Repashy, etc.
In the summer I also set out a bucket of water and collect the mosquito larvae to feed to my fish. Man do they go ape over them! They perk up so much that I don’t even recognize their behavior!!
So, like I said, variety is important as no one food can provide a complete balanced diet for any creature.