Can neon tetras be in the same tank as a Siamese fighter fish?

2 Tetras = 2 gallons, 2 mollys = 6 gallons, 2 guppies = 2 gallons, 1 Betta = 3 gallons. Provided your tank holds a bit more than 15 gallons of water (about 20 gals) and your filter will handle the job, there is no reason I can think of that would stop you putting a Siamese Fighter (Betta splendens) in the tank with the fish species you mention.