After your tank is well cycled and established, it would be best to add the cories so you can quarantine them before adding the betta.
Corys do best on sand, but VERY smooth pebbles (about a 2" layer) can be used and plants put into that.
You can also tie some Java fern and anubias to rocks or
driftwood. They're both low light plants and you could put some cryptocornes directly into the substrate.
Pandas like cooler temps than bettas do, so you might be better off getting another kind.
4 of them might be crowding things a bit, unless you get a smaller variety.