Are you or your dad experienced in fish at all? Neon tetras are one of the most fragile fish you can buy. They probably won't survive the cycle.
You really should have waited until you found out the personality of the betta. You can mix aggressive and placid tropicals, but you'll want to have a tank that is big enough and a stocking plan that doesn't cause much stress to the small fish. Tetras are a schooling fish and you should always have more than a "couple" in a tank with a potential predator. Platies can do fine in that water. I consider them a coldwater fish but they can live in trop tanks.