Every betta is different when it comes to who they will and who they won't accept. My male betta gets along fine with other fish in the tank, while my female is much more aggressive. The male gets along great with platies and corys. Some tetras may be okay, but others will nip at the fins of a betta. I've never kept tetras myself, so I couldn't really say which were safe. A lot of the smaller fish are schoolers, so you'll most likely be limited to the betta and a small school of one other species. You could also try african dwarf frogs, mystery snails, or ghost shrimp.
As far as plants go, java fern, anacharis, and Hygrophila polysperma are pretty easy. The type of plants that will grow in your tank really depends on the lighting you have. Don't try to get away with high light plants unless you have a good lighting system, they'll just die eventually and polute the tank.