What size is the tank? I hope it is at least 20g with your current stocking.
The biggest problem I see with this mix-is feeding the Betta-you have two species of fish that will out compete him for food....Platies can sometimes nip fins and if you have platy fry the Betta will enjoy them as a snack.....
Filtration can also be an issues-if it is too strong the Betta may have problems with that as well and you can see fin damage, hiding and/or appetite loss-the younger the Betta the more tolerant they tend to be with water flow/current.
Also, you need to add to your cory cat school...at least 4-6 and 8-10 is better all depending on the tank size.