Of course. Pretty much everyone eats them, but I would venture to say that gouramis and bettas eat them more readily than other fish do, simply because it's a great deal of their "natural" food source in the wild.
I used to have a couple kissing fish and they ate them right up. The Blue Gourami I used to have also ate them down pretty good. I now have a couple Dwarf Gouramis and they tend to eat them pretty good.
No they don't but it will make them happier as with any other fish. Plants also will help with certain water prams. Simple sword plants are good to have as they tend to be pretty easy to care for. Most Gouramis like broad leaf plants as they will use them as cover. I myself have plants tword the sides of my tank and in the middle is more open to allow for swimming room.