Haha "eat poop" is the simplest way to put it, the poop gets broken down into various nutrients that then get absorbed by the plants, such as nitrogen, phosphorus and potassium.
With the guppy, I believe the gouramis would consider it a yummy snack unfortunately (perhaps I am wrong about this, I know they love to munch guppy fry as tasty treats) and they are schooling, so they are happiest in groups of 5 or more, and you have to be careful to get all one gender unless you want to have a guppy fry infested tank (which, again with gouramis I doubt will happen)