Hikari makes a good food called "First Bites". It's better for livebearer fry than egglayers so right up your alley.
As the fry grow, there is another food by Hikari called "Tropical Fancy Guppy". That is better food for growth, conditioning and breeding. It should be used in addition to other foods like frozen baby brine shrimp, spirulina flakes, blackworm flakes.
Another good choice is Dr. Bassleer Biofish food baby nano. It's a protein packed food for growth but it is a bit expensive. ( Good food ain't cheap.
) Northfin also makes a fry food but I don't believe it comes in smaller sizes so also a bit expensive.
As for feeding, baby fish need more feedings than adult fish so at least 2-3 times per day. Look at their bellies before you feed. If they are round, they don't need feeding. If they are flat, they need feeding.
I would not feed pellets to Guppy fry unless you crush them into a powder first. In a pinch, you can crush regular flake food into a powder but adult foods are more for sustaining life not good growth so they should not be the main food fed to fry.
Lastly, fry foods are going to make the water foul if you over feed ( and you will over feed because there is no standard amount to feed ) so you should be doing water changes on fry tanks at least 2 times per week ( especially in a 6.5 tank. )