Bettas don't really need or in most cases appreciate tankmates. That's more for our own enjoyment, which is fine as long as their are no ill effects for the fish. Whether a betta will tolerate tankmates differs from betta to betta. I have 4 cories in a ten gallon with a betta and the only time he notices them is when he thinks they might have food. Just make sure you watch for at least an hour after you introduce them to make sure that your betta doesn't care about tankmates. If possible, put the cories in first, so they have time to find all the hidey holes and then put in the betta. If he goes after them, take him or them out right away before the cories get hurt.