Hello Eval
For the Guppies, I would suggest using Omega One Super Color pellets or flakes. I prefer pellets because they don't break in the container, and they won't foul up the water like flakes. Plus, the color enhancement will bring out the colors of your fish! You can also feed them blanched peas, frozen blood worms, and Omega One Freshwater Community Formula frozen food. Pellets/flake should be fed 1-2 times a day, while peas, other veggies, and frozen foods should only be fed 1-3 times a week.
For the Corydoras, you can feed them a variety of Omega One Shrimp Pellets, Veggie Rounds, blanched peas, frozen blood worms, Hikari Sinking Waffers, and any thing else you feed your Guppies that slips past them
For the Shrimp and Snails, I would suggest feeding Hikari Crab Cuisine, Omega One Shrimp Pellets, Omega One Veggie Rounds, blanched Peas, frozen bloodworms, Hikari Sinking Waffers, and they will feed off some algae and bio-film found on live plants. You can also feed them (and your other fish) blanched spinach, cucumber, and zucchini. There is also a Hikari Shrimp Cuisine you can feed. Some people like to feed their shrimp expensive shrimp food that contains all sorts of stuff, even pumpkin! I don't feed mine those expensive 'designer' foods.
Now you will notice I suggested only Omega One and Hikari.
If you look at the ingredients of Omega One, you will find lots of whole foods. For example, lets take a look at the Omega One Super Color Pellets I suggested.
Ingredients are:
Whole Salmon, Halibut, Seafood Mix (Including Krill, Rockfish, & Shrimp), Wheat Flour, Wheat Gluten, Fresh Kelp, Astaxanthin, Lecithin, L-Ascorbyl-2-Phosphate (Source of Vitamin C), Natural and Artificial Colors, Vitamin A Acetate, Vitamin D3 Supplement,Vitamin E Supplement, Vitamin B12 Supplement, Riboflavin, Niacin, Pantothenic Acid, Folic Acid, Biotin, Inositol, Tocopherol (Preservative), Ethoxyquin (Preservative).
Yes, it contains a preservative, along with a few other items, but it is much better than the typical TetraMin flake you see that is mostly fillers and by-products. I have switched from TetraMin to all Omega One, Hikari, and fresh veggies and I have noticed the fish have been more colorful, active, and the food seems to fill them faster than the TetraMin Crisps I was feeding before.
Hikari by all means is not top of the line, but they DO make good food, and that is why I include it in my fishes diet. Omega One and Hikari are a bit more expensive than other foods, (except for New Life Spectrum, which is another great one, but it is expensive so I choose to feed Omega One and Hikari which is just as good IMO) but way worth the quality of it.
Now I can joke my fish eat better than me!
Hope that novel helps,
Scotty
*Edit- Please not, you do NOT have to feed all these different types of foods if it sounds to complicated. It's not, but as long as you feed with a quality pellet/flake with some frozen blood worms and blanched veggies a few times a week, your fish will be fine. I just prefer (and I know a lot of different fish keepers out there besides myself) feeding many different types of foods.