When you return the betta, move the plants and decorations around prior, so it will be like a new tank to him. He will work to establish his territory and may chase any fish off that gets too close.. or he may calm down.
Each betta has their own personalities.. some do great with tank mates, while others don't like small ones, or fast ones, or brightly colored, or any at all. It's a gamble when setting up a community tank with a betta, just never know.
So give him a little more time in there, if he isn't calming down either give him his own tank, or take the 2 platies back is my suggestion.