I currently have an Eclipse tank with both a betta, an otto, and ghost shrimp.
When the otto first went in my betta wouldn't leave him alone. He followed the otto everywhere and flared at him constantly. After about a week he lost interest, now he ignores the otto. I never saw any evidence of him attacking the otto, I think it was mostly curiosity and making sure the otto knew who was in charge.
My ghost shrimp went in recently with much the same effect. I didn't know how he'd react to them or even if he'd try to eat them but at 36 cents a pop I figured no big loss if he did eat them. He's still curious but so far no attacks.
Watch my tank progress from Craigslist salvage to fishy habitat: Aaron's Tank
The only things that happen quickly in an aquarium are bad things.