If you're going to get Pygmy cories, they need a cycled, well-established tank. The only reason I happen to have this information is I've been Googling the heck for them and that's the one thing on which all catfish forums agree.
You can add them after the Betta but rearrange the tank. When I introduce mine, I will float the Betta in an opaque cup so he can't see them floating in the clear bags and get any ideas. I will release them into a darkened tank and then release him.
Others do it differently, I'm sure. That's just my way.