The idea of Creo for nutritious mushrooms was coming from the fact that deadly mushroom would need a Perdo requisit, and I thought that nice, nutritious mushroom might need a Creo requisit, since between the delicious one and deadly one, there are mushrooms that are just not suitable as a source of nutrient without being toxic either.
Frogs are edible (I can confirm, have tested experimentally on a few occasions) and the spell to create them doesn't require a Creo prereq.
There is no Creo requirement to create Vermin. Many types of Vermin, especially water based ones, were consumed as a food source. Eels, frogs, snails, crayfish, river mollusc, etc. There are also several places around the world that have traditionally eaten mice or rats, though none in ME. Mice are a Vermin and can be created.
If your group classifies mushrooms are a Herbam version of Vermin, then there would be no Creo requirement in spells to create them. However it would require the spell be cast on the proper base material and it is well known at the time the materials that many specific types of mushrooms grow from.
You could also create Vermin as a food source booster rather than a food source. Creating Honey Bees (which come from the corpses of cattle RAW) would both boost pollination as well as providing a source of honey and wax. Creating water insects would serve as a food source for pond fish, allowing their population to boom.