If you were to get a ten gallon you could have tankmates. I recommend cories because they are bottom dwellers and unlikely to draw attention from the betta. If you get bronze/green/albino cories you can have four in a ten gallon. You should not do less than four of the same species though. If you got pygmy cories you could 6-8 in a ten gallon. Other options include neon tetras, platys, endlers, some others. I've only ever kept cories with bettas though so I'm not much help with how many of the other species to keep for a school.
In terms of plants you can use plastic plants that won't damage fins, silk plants, or live plants. Some oftern recommended live plants are anacharis, anubias, java fern, java moss, and marimo moss balls. There's a sticky on the housing section about live plants for more information.