The tetras and cories are schooling fish and really need to be in groups. Schools of 5-6 are what to aim for. Seeing them in large schools is definitely a treat, and fish that live in schools like this can become stressed and eventually die if they aren't in proper schools.
Your tetra looks like a white skirt tetra. Getting more of the same or possibly some black skirt tetras (they can't tell the difference) would be a good idea, especially as these fish are known to be nippy and increasing the school size can cut down on this. They won't school with other species of tetras, though, only with different color morphs of their own species.
Although some cories are known to school with other species, it's probably safer to just get more of the same species. As others have said, that rough gravel will really be hard on their barbels and after the tank is established, you should consider replacing the gravel with sand. It looks great, too.
The gourami should do a pretty decent job of eating your
Guppy fry should you have mixed sexes and thus a lot of breeding going on.
EDIT: That'll teach me to post a response before reading Page 2 of the thread. Good to hear you've increased the school sizes, although three or more cories wouldn't hurt. About their barbels: corydoras catfish are scavenging bottom-feeders; they use their sensitive barbels to feel for bits of food along the bottom. With a rough substrate like yours, the barbels will wear down and won't allow them to find food as effectively. Also, they can cut or otherwise injure their barbels which results in infection, which then often leads to death. Switching substrates is definitely good idea but is best done once the tank has matured and should be done in increments in order to avoid a mini-cycle.
