If you do get corydoras, they do best in groups so I think six would be a good number in your tank. Some will school with other species of corydoras and some will not, so to play it safe I would just get six of the same species. There are a lot of different species so have a look at them and choose the ones that you like best. Be forewarned that there are a few species that don't really fit the general corydoras guidelines and might not be suitable for your tank, but these are a minority.
Although they are bottom feeders, cories will eat just about anything so you should feed them a nice balanced diet. The wafers will work but they'll also eat flake food that has sunk to the bottom, sinking crumbles, shrimp pellets, and just about any frozen or live foods that are small enough for them to eat and that sink to the bottom.