I would not put tetras with a Betta as they will most probably go after the Betta's fins.
I have never seen Rasboras or Cory cats ever bother any other fish.
I had put 6 black neon tetras in with my Betta, a Malabar Puffer and a deformed guppy...it lasted 1 week before I had to move the tetras...they were going after every one...my Betta, Puffer and Guppy are all slow moving fish and the neons were trying to nip all of them. They were in a 30 gallon tank, so there was plenty of room for everyone.