Corys are fine. Mine lived steadily with Cory Cats for quite a while before I moved the Corys to my Guppy/Molly/Platy tank.
Guppies...it sometimes works, and it sometimes doesn't. One of my male Bettas would literally snuggle in the plants with one of the female Guppies, and one of my female Bettas would flare at them ONLY if they made aggressive moves toward her...other than that, she swam around with them just fine. But one of my male Guppies had a problem with my male Betta and nibbled his fins (go figure, the little one is the evil one). And, in the other case, one of the Mollies bodily attacked my female Betta, and I found the female Betta in a corner of the tank, being nipped at mercilessly. Another female I tried in the Guppy tank (before the addition of the Mollies) was in the tank less than three seconds before reaching over and very rudely removing the tail of my male Guppy. She didn't stay in that tank more than a total of 6 seconds, because she was going after the nearest female Guppy, too.
My Platies are Dwarf Platies, so they are considerably smaller (especially since they're juvenile) than any Bettas, and I never had a problem with them. They mostly did their best to stay WAY out of the way, but I don't really know how a normal Platy would react to a female Betta.